Democratic Senators Kyrsten Sinema and Joe Manchin are dominating Capitol Hill. Given the 50-50 split in the Senate, Democrats can’t afford to lose a single vote on the budget reconciliation bill — and Sinema and Manchin are using that leverage to force huge concessions from the Biden administration.

Both Sinema and Manchin are insisting on paring down the overall cost of the $3.5 trillion budget reconciliation bill, which (in its original form) would have drastically expanded the social safety net and bolstered America’s fight against climate change. Sinema has resisted raising corporate tax rates (which were lowered in 2017 under President Donald Trump) to help finance the bill. Manchin, on the other hand, has made it clear that he opposes a key measure that would accelerate the transition from fossil fuels to clean energy.

This week, President Joe Biden is starting to intervene more directly as negotiations drag on. But it’s unclear how much headway he can realistically expect to make. Despite decades of experience in Washington, he faces a dysfunctional Senate that has been punting for many years on issues including immigration, climate change, criminal justice reform, childcare and more.

Given the way the deliberations have unfolded since the summer, Sinema and Manchin seem to be holding fast while the rest of the party is scrambling to meet their demands. As a result, because of the centrality of the budget bill to the entirety of Biden’s domestic agenda, these two senators have acquired an overwhelming influence over the future of the Democratic Party, the legacy of Biden’s presidency and the direction of our country at this pivotal moment.

To be sure, we’ve seen instances in the past when a single senator had the potential to decide a political party’s fate. In 1952, Sen. Wayne Morse of Oregon broke with the GOP and declared himself an independent when Dwight Eisenhower selected Richard Nixon—then considered to be one of the fiercest Cold Warriors—as his vice president. While Morse continued to caucus with Republicans, he was stripped of his seniority and removed from key chairmanships. When he joined the Democrats in 1955, it allowed the party to gain control of the chamber with a one-vote margin.

In 2001, Sen. Jim Jeffords of Vermont broke with the GOP and decided to caucus as an independent in a move that shifted control of the 50-50 Senate back to the Democrats. “Mr. Jeffords Blows Up Washington,” a Newsweek headline announced at the time. While he temporarily checked Republican ambitions to roll back federal policy, the September 11 attacks shifted the agenda to national security and the GOP went on to regain control of the upper chamber in the 2002 midterms.

In 2017, Sen. John McCain single-handedly changed his party’s agenda when he gave a literal thumbs down and ended the years-long Republican effort to repeal the Affordable Care Act.

While the fate of the budget reconciliation bill rests on Sinema and Manchin, Democrats are not just reckoning with two unruly senators — they are confronting the problem of the Senate itself, which gives small states disproportionate power over key issues. Think of the tens of millions of people who might suffer from the climate crisis because a single senator from West Virginia, who has profited from the coal industry, has the power to block measures to combat it. Because the Senate gives equal representation to big and small states alike — and retains practices like the filibuster — small minorities can ruthlessly obstruct larger voting blocs.

This dynamic was evident when the Southern Democrats ruled the roost from the 1930s to the 1960s. Georgia’s Richard Russell was a force unto himself. Even as the nation was changing dramatically and a large portion of the Democratic Party called for more robust federal programs to deal with issues such as health care and civil rights, Russell led the members of the Southern Caucus to use the filibuster and their control over committees to block progress on these issues.

They reshaped the party’s legislation by allowing certain elements of federal policy to pass through the chamber — but only when certain provisions related to race relations and union power were dropped. Now, Sinema and Manchin are playing a similar role, with the key fault lines revolving around climate change and taxation.

It’s also worth noting that Biden’s agenda hinges on Sinema and Manchin in part because of the unyielding nature of the GOP. This is, of course, Minority Leader Mitch McConnell’s doing. And while the infrastructure bill has found bipartisan support in the Senate, Congressional Republicans have taken a firm stance against the administration, forcing Democrats into a difficult position with few options on the table. With little hope for any dissenting Republican votes, Democrats need to use reconciliation to pass the Build Back Better agenda with a simple majority.

Americans today also live in an era, as Princeton political scientist Frances Lee has argued, when control of Congress is less stable than it once was. Gone are the days when one party reigned over Congress for long periods of time, as Democrats did from 1955 to 1994 (with the exception of the Senate from 1981 to 1987). This means that neither party feels secure about its future, which might lead to fewer policy risks due to a fear of upending the power balance.

For Democrats, there are two ways to begin remedying the problem. One is reforming the Senate. The most urgent priority would be to end or reform the filibuster. When the right to filibuster is considered more sacrosanct than the right to vote, it’s clear the Senate has a serious problem on its hands. Biden himself said on Thursday he was open to making changes to the filibuster.

The other solution is for Democrats to double down on efforts to expand its reach, undertaking the kind of grassroots initiatives that activists like Stacey Abrams spearheaded in Georgia to switch the state from red to blue. Once these elections are over, these activists need to maintain pressure on the winner to consider the overall health of the party rather than his or her own future.

Editor’s note: Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the book, “Abraham Joshua Heschel: A life of Radical Amazement.” Follow him on Twitter @julianzelizer. The views expressed in this commentary are his own.

President Joe Biden takes off his face mask before addressing the 76th Session of the United Nations General Assembly, Tuesday, Sept. 21, 2021, in New York. (AP Photo/Evan Vucci)
President Joe Biden takes off his face mask before addressing the 76th Session of the United Nations General Assembly, Tuesday, Sept. 21, 2021, in New York. (AP Photo/Evan Vucci)