On Tuesday night, progressive Democrats in New York claimed another victory in their effort to move the party left by toppling establishment-friendly incumbents, as Jamaal Bowman, who was endorsed by Alexandria Ocasio-Cortez, Bernie Sanders and Elizabeth Warren, claimed victory in his primary challenge to longtime Rep. Eliot Engel.

High-profile primary fights such as this one are energizing and important, but on their own, they won’t determine the direction of the Democratic Party. Instead, ideological change — if it comes to the party at all — will likely be achieved through the less hyped process of political churn as politicians retire and through the self-defensive “evolution” of sitting elected officials on key issues.

Highly covered primary fights — including the ones raging in New York — make for flashy news stories, but they are exciting precisely because they are rare and, as a result, don’t do much to change the overall composition of the party. Beating an incumbent congressman — like Ocasio-Cortez did and Bowman seems on track to do — is highly difficult. Most Americans think their congressman deserves reelection, and sitting House members often have the money and institutional backing to repel challengers. Ocasio-Cortez and Ayanna Pressley of Massachusetts were the only two candidates to successfully primary sitting House Democrats in 2018, a year when anti-Trump sentiment should have helped progressive candidates like them. That’s two seats out of almost 200 that were held by Democrats heading into that election.

And Ocasio-Cortez and Pressley won safe Democratic seats, where the outcome of the primary determined the outcome of the election. The impact of primary fights on the party’s positions is even more limited in states where any Democrat would face a steep uphill climb to beat a Republican. That’s the case in Kentucky, where the primary between Amy McGrath, a retired Marine fighter pilot, self-described moderate and prolific fundraiser, and Charles Booker, a progressive African American state legislator became a national story, despite the fact that neither is likely to beat Senate Majority Leader Mitch McConnell in the fall.

The normal churn of the political system alone provides more opportunities for progressive gains.

In the 2018 election, 18 House Democrats didn’t seek reelection. This time, the number is closer to 10. Every single retiree needs to be replaced — and as the tea party demonstrated in the 2010s, outsiders can flourish in these wide-open races. Remember that many early stars of the conservative insurgency — Marco Rubio, Ted Cruz, Scott Walker and Ron Johnson, among others — started on their path to national prominence by pursuing open seats rather than knocking off an unsuspecting incumbent. Warren, Katie Porter and other high-profile progressives took that same path to Congress.

But ultimately even generational replacement and well-placed candidates in open-seat primaries will do only so much. If Democrats move left, it’ll be partially because their sitting politicians move themselves, in part to fend off their challengers.

Just consider the case of Joe Biden: for his entire political career, Biden has positioned himself in the center of the Democratic Party. When the Democratic Party was interested in waging a disastrous “war on crime” and mostly in favor of the Defense of Marriage Act, which was intended to block same-sex couples from marrying, Biden was on board. But when the party shifted, he did, too. He came out in favor of same-sex marriage in May 2012, before President Barack Obama did, and now says he wants to fight the institutional racism he helped enact earlier in his career.

Kirsten Gillibrand is doing the same thing in real time. When Gillibrand was a congresswoman from New York, she opposed “amnesty” for undocumented immigrants and slept with two guns under her bed. But when she decided to run for Senate in New York — and later prepared to run for president — she moved left to match her new base.

This process of ideological change is quiet, sometimes halting and less emotionally gripping than our epic narratives about primary challenges and unseating entrenched establishment hacks. But ultimately, this is how parties really move: New leaders are rotated in through the most pedestrian processes, and old leaders adapt to new constituencies in an effort to keep their own jobs secure. It’s messy, boring and tedious to track — but in the best cases, this system allows parties to understand and respond to voters without burning down institutions every two years.

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