Fights over political lines have fueled the aging of America's government. But that may be changing.

• Redistricting and fights over political lines have fueled the aging of America's government.  
• More often than not, older members of Congress win in primaries between two incumbents.
• But polarization and partisanship are enabling younger members to knock out their elders.
• Read more from Insider's "Red, White, and Gray" series.

Every 10 years, a new US Census forces states to redraw their political maps through redistricting. Whenever a state loses a House seat in the process, incumbent members of Congress are forced to run against each other — and younger, less experienced incumbents often end up on the chopping block. 

An Insider analysis, based on data compiled by Bloomberg's Greg Giroux, found that in 20 incumbent-on-incumbent contests since 1992 where there was an age difference of at least two years between the candidates, 12 were won by the older member and eight by the younger member. 

The dynamics of member-on-member contests have favored older incumbents and reinforced the gerontocracy in American politics as we know it. Since the 1990s, the age of Congress has significantly increased compared to the baseline growth seen over the 20th century — and redistricting has only hastened that trend. 

"It doesn't surprise me that age would make a difference on the margins," Jamie Carson, a political scientist at the University of Georgia who studies congressional elections, told Insider of member-on-member races. "It brings a bit more experience, better fundraising prowess, and maybe they know the district better." 

The 2012 cycle, where the more senior representative won eight of 11 such contests following redistricting, was particularly brutal for younger incumbents. 

In New Jersey, then 75-year-old Bill Pascrell easily dispatched 59-year-old Democratic Rep. Steven Rothman by 22 points when the two ran in the same district in 2012. 

Fifty-year-old Rep. David Schweikert narrowly ousted the one-term 35-year-old Ben Quayle, the son of former Vice President Dan Quayle, in the Phoenix suburbs. Sixty-nine-year-old John Mica, who had served in Congress for nearly 20 years, knocked out 55-year-old Sandy Adams, who served one term, in suburban Central Florida. And 60-year-old Janice Hahn defeated fellow Democrat Laura Richardson, 50, in the general election for California's 44th District by a resounding 20 points.

Youth movement — fueled by partisanship?

But that trend is changing. Accelerating partisanship and polarization, in particular, are shaking up the game — and increasingly empowering younger lawmakers to knock out their more senior rivals. 

In 2022, the 75-year-old, five-term Rep. David McKinley, a West Virginia Republican, had some key advantages on his side when reapportionment put him in the same district as 50-year-old Rep. Alex Mooney. 

McKinley, endorsed by both Virginia's Republican governor and its powerful Democratic senator, Joe Manchin, was able to tout his role in the passage of a landmark bipartisan infrastructure bill that provided much-needed funds for his state. But an endorsement from President Donald Trump and hundreds of thousands in outside spending from conservative groups boosted Mooney to win the day. 

Younger candidates have won four out of six member-on-member primaries of the 2022 cycle, though in one case, Rep. Jerry Nadler of New York, who's 75, was younger by just a few months than Democratic rival, Rep. Carolyn Maloney. And in the cases of the two older incumbents who came out on top in their races, age and tenure in Congress weren't the deciding factors.

State lawmakers have expertly wielded their redistricting pens to kneecap their opponents — and hurt younger candidates 

Over the decades, intentional partisan gerrymandering — and the inevitable reshuffling that comes along with states losing districts and redrawing their lines — have dashed many congressional careers and aspirations.

Partisan gerrymandering and internal party struggle over district lines can also prevent young candidates or candidates from diverse backgrounds from getting elected to Congress in the first place. 

"Wisconsin's 7th District is very gerrymandered, so that was already a challenge that, going in, I knew," Tricia Zunker, who ran as a Democrat in a special election in a rural, Republican-leaning north-central Wisconsin district in spring 2020, told Insider in an interview.

Partisan gerrymandering, the redrawing of political district lines to favor one political party over the other, has gotten increasingly tactical. State lawmakers, in addition to utilizing the process to kneecap the opposing party, also make strategic choices about which members of Congress to sacrifice. 

In 2002, Republican state lawmakers in Michigan, who then controlled the redistricting process, drew two Democrats into the same blue district to make the surrounding districts safer for their party, a gerrymandering tactic known as packing.

The new map put then 45-year-old Rep. Lynn Rivers in the same district as Rep. John Dingell, a scion of Michigan politics who still holds the mantle of the longest-serving member of Congress in history.

Rivers mounted a competitive challenge — but still lost the race by 18 points. And she's not the only one who saw her time in Congress come to an end after partisan gerrymandering pitted her against a colleague. 

After the 2020 Census, Republican lawmakers in Georgia took the opportunity to force out one of the two Democratic women who had won seats in the blue-trending Atlanta suburbs, pushing Reps. Lucy McBath and Carolyn Bourdeaux into the same safely Democratic district. 

In Illinois, which lost one House seat after 2020, a map drawn by Democrats in the state legislature pitted two sets of Republican incumbents against each other in northern and central Illinois. Rep. Rodney Davis lost to freshman Rep. Mary Miller in one of those races. In the other, Rep. Adam Kinzinger decided to retire rather than run against Rep. Darin LaHood. 

"Through a strategic lens, it's a chance to knock out an incumbent and make that seat more vulnerable," Carson, the political scientist, said of gerrymandering.

The game of political musical chairs set off by a state losing a seat in Congress can also box out young, upstart candidates hoping to break in. 

Lourin Hubbard, a 33-year-old water-quality-control manager in California's Central Valley, ran as a Democrat in a June 2022 special election after a top GOP representative, Devin Nunes, left Congress early to run the Trump Media & Technology Group, the company behind the former president's Truth Social platform.

Hubbard, who lost to the 71-year-old Republican Connie Conway, considered running for a full term in Congress. But California's new commission-drawn congressional map, which will fully take effect come January 2023, dissolves Nunes' old district and splits it up into four new ones — including one held by Democratic Rep. Jim Costa. 

Hubbard told Insider that local county Democrats discouraged him from running for a full term in Congress against Costa, who now holds a safely Democratic Fresno-based seat.     

The new map "created some tension and conflict" within the county Democratic Party. "And to avoid that conflict altogether, I was like, 'I'll just run in the special,'" he said. 

Partisanship and parochial factors are shaking up member-on-member primaries

Incumbency is the best predictor of whether a candidate wins an election. 

Challengers, Carson noted, have the best shot at unseating incumbents if they have prior political experience, money, or are up against an incumbent with particular weaknesses or a scandal.

"Even if you don't like the incumbent, that's not enough," Ryan Williamson, a resident fellow for governance at the R Street Institute, a Washington DC-based policy think tank, told Insider. "You have to actively dislike the incumbent and mobilize around unseating them, which is a much higher burden."

But the rare contests where incumbents are pitted against each other are "idiosyncratic," Carson said, and don't follow predictable patterns — especially in the 2022 cycle.        

Two decades after the Dingell-Rivers race, another primary fight between two Michigan Democrats shows how partisanship and money can eclipse age, experience — and even membership in a political dynasty — in member-on-member races. 

Rep. Haley Stevens, 39, could have had an uphill battle against Rep. Andy Levin, 61, when he decided to run against her in her suburban Detroit district, which became more safely Democratic under new lines drawn by a citizen-led commission. 

Like Dingell in 1955, Levin took over the House seat held for decades by his father, Sander Levin, in the 2018 midterm elections, following both his father and his uncle, former Sen. Carl Levin, to Washington. 

But Levin took a risk by choosing to run in the safely blue 11th District instead of the more Republican 10th District where he lived, planting himself on largely unfamiliar turf where he represented just a quarter of the electorate. 

The explosion in outside campaign spending since 2002 has shaken up the dynamics of all House primaries and heavily factored into the 11th District contest. A slew of pro-Israel, women's, and progressive organizations poured over $9 million, $7.45 million of which benefited Stevens, into the race. Stevens ended up beating Levin by 20 points in August — overcoming both his seniority in age and the potency of his family name. 

Like in all House elections, money plays a significant role in member-on-member races. Older incumbents often have more money in the bank and more established fundraising networks from their years campaigning for elected office and, in some cases, greater personal wealth, candidates and experts told Insider. 

The candidate who spent the most money won four out of six member-on-member contests in 2022 and nine out of 11 contests in the 2012 cycle, according to an Insider analysis of campaign-finance records. 

"Every new midterm breaks the previous record, and a bulk of that money is going to flow to the most competitive primaries and general elections," Carson said, noting that "money is a necessary but not sufficient condition," particularly in member-on-member contests. 

State lawmakers have drawn incumbents together for political gain for decades, but the post-2020 redistricting cycle also saw many maps drawn by courts or independent commissions. Many of those commissions prioritized drawing competitive districts over protecting longtime incumbents, which blunted the advantage that some more tenured incumbents enjoyed. 

New York's court-drawn congressional map, resulting from a messy and protracted legal battle, pitted two longtime House Democrats and committee chairs, Nadler and Maloney, against each other in upper Manhattan.

That race, a clash of two septuagenarian titans of New York politics, was driven more by parochial forces than national ones, with The New York Times editorial board and the powerful Sen. Chuck Schumer throwing their support behind Nadler in the final stretch. 

As the Stevens-Levin race showed, the member of Congress representing most of the new district's constituents is generally favored in a matchup between two incumbents in the same seat. 

But in other races, like the Hahn-Richardson contest, a candidate who represented very little of their new territory came out on top.  

And four out of six member-on-member contests that have taken place so far in 2022 saw the member who represented less of their new district winning their race, according to Daily Kos Elections. In Georgia, for example, McBath won despite representing just 12% of her new district, while her opponent represented over 57% of it.    

Partisanship and ideology are now playing a more prominent role than age and seniority in shaping House primaries, especially on the Republican side. 

The winners of the two 2022 primaries where Republican incumbents faced each other, Miller and Mooney had firmly aligned with Trump's wing of the Republican Party, earned Trump's endorsement, and also enjoyed outside spending on their behalf by groups like the Club for Growth and the political arm of the House Freedom Caucus. 

In Illinois, Miller defeated Davis by 14 points despite raising and spending less money than Davis, serving in Congress for less time, and only representing slightly more of the new district. Mooney, who also served less time and represented just a third of the redrawn district, defeated McKinley by an even wider margin of 18 points. 

Knocking out an incumbent with years of lawmaking experience is still difficult, and older candidates' structural advantages aren't disappearing. But the forces of partisanship are making it easier for challengers. 

House Republicans who fought the Trumpian takeover of the party, like Reps. Liz Cheney, Tom Rice, Peter Meijer, and Jaime Herrera Beutler, all lost to Trump-backed primary challengers. And others, like Kinzinger and Reps. Anthony Gonzalez, John Katko, and Fred Upton, chose to forgo running altogether. 

In all, 14 House incumbents lost renomination in 2022, the highest number in a single cycle since 1992. The forces of partisanship and polarization, Carson said, would have pushed that number even higher if not for dozens of members heading for the exits.

"For the longest time, it was hard to beat incumbents because people of the opposite party would be willing to cross party lines," Carson said. "That has changed. If you're a Democrat in a Republican district, your days are probably numbered."