The Democratic primary is full of big ideas. Candidates are vying to offer the most ambitious plan to combat climate change. They’re sparring over dramatic changes to the health care system and proposing sweeping gun control legislation. But they keep running up against the same question: How are they going to pass their policy agenda through the Senate?

It’s a conundrum that has thrust the Senate’s rules and procedures — specifically, the fate of the legislative filibuster, which requires a supermajority to end debate on a piece of legislation — into the center of the Democratic primary. Because even if Democrats manage to win both the White House and the Senate in 2020, the 60-vote requirement to break a filibuster could easily allow Republicans to halt some of the Democrats’ biggest policy proposals.

In response, the call to end the filibuster is turning into a rallying cry on the left. Progressive activists, who see Republicans’ use of the filibuster to obstruct President Obama’s agenda as a cautionary tale, are now pushing primary contenders (and even senators) to get rid of it. And increasingly, the candidates seem open to the idea — although like most structural changes in government, killing the filibuster is more easily said than done.

How the filibuster could be a barrier to Democrats’ policy agenda

Over the past 10 years or so, the filibuster has increasingly become a weapon of obstruction for the minority party. As a result, its enemies now include both President Trump and Sen. Elizabeth Warren, who have argued that it gives the minority an unfair veto over the majority’s agenda. And it’s true, at least, that filibustering has become more and more routine. And as the chart below shows, the number of times senators have filed a motion to invoke cloture — the first step in ending a filibuster — has more than doubled between George W. Bush’s presidency and Trump’s presidency. It first spiked dramatically in 2007, when Republicans lost their Senate majority in the 2006 midterms and began using the filibuster much more aggressively than either party had in the past. The trend only accelerated after Obama took office in 2009. “We’re now in a situation where you really need 60 votes in the Senate to get anything done,” said Josh Chafetz, a professor at Cornell Law School who studies Congress.

The filibuster’s defenders argue that it remains an important tool that forces negotiations between the majority and minority parties. But this hasn’t stopped both parties from weakening it. Frustrated by the Republicans’ refusal to confirm Obama’s nominees to the lower courts and executive branch posts, Democrats got rid of the filibuster requirement for those nominees in 2013. Four years later, the Republicans ditched the 60-vote threshold for Supreme Court nominees to confirm Neil Gorsuch.

Now, the legislative filibuster is the last vestige of the minority’s obstructionary power, and it could turn into a serious roadblock for Democrats if they win control of the presidency and the Senate. “The lesson of the past 10 years or so is that minority parties have realized it’s in their interest to block whatever the majority is doing,” said Gregory Koger, a political science professor at the University of Miami who studies the filibuster.

This tendency toward obstruction probably hurts Democrats more than Republicans, experts said, because Democrats are more likely to campaign on big, sweeping changes to the system that require legislation to become reality. Sarah Binder, a political science professor at George Washington University and an expert on Congress, pointed out that earlier in the 20th century, the filibuster was mostly used by southern segregationist senators to stymie anti-lynching bills and civil rights legislation. “The history of the filibuster is not especially favorable to liberal causes,” she said. “Arguably, it’s in the Democrats’ interest to get rid of the filibuster because they’re more likely to pursue activist solutions to policy problems.”

Progressive activists are pushing the candidates on filibuster reform

Some liberal activists think the filibuster is an existential threat to the bold policies that are being debated in the Democratic primary. “The filibuster is about the future of American democracy,” said Ezra Levin, the co-executive director of Indivisible, a national progressive movement that recently launched a campaign against the filibuster. “If it exists in 2021 and 2022, it won’t matter if the Democrats have won the presidency and the Senate — all of those progressive proposals we’re debating will be defeated and stalled.”

And in the last few months, killing the filibuster has gone mainstream. When their campaigns were just getting off the ground, many of the candidates — particularly the senators — seemed leery of getting rid of it. But Warren and Washington Gov. Jay Inslee both started openly calling for the filibuster’s demise, and other candidates appeared to warm to the idea, too. According to The Washington Post, 12 active presidential candidates that FiveThirtyEight considers “major” have either said that the filibuster needs to go or that they’re open to eliminating it.

The issue even came up briefly in the third Democratic debate, when Warren and Sen. Bernie Sanders disagreed about the right way to circumvent the 60-vote requirement. So far, Sanders has opposed getting rid of the filibuster, but his solution for avoiding a filibuster is also quite radical. He’s proposed ramming his signature legislation, Medicare for All, through the Senate using the budget reconciliation process. That would only need 51 votes for passage, but it would still involve a dramatic reinterpretation of Senate rules and leave the door open for other non-budgetary proposals to be passed the same way. “Sanders might be trying to reassure voters who think ending the filibuster is too extreme, but what he’s proposing would effectively get rid of the filibuster,” Chafetz said.

But killing the filibuster is easier said than done

None of this means, however, that the filibuster’s fate is sealed. For one thing, filibuster reform won’t be possible for the Democrats unless they have the White House and the Senate under their control. And even if that happens, supporters of ending the filibuster will have the not-so-simple task of convincing Senate Democrats to give up a power that is undeniably helpful to the minority. Sanders, in particular, has cited Trump’s eagerness to end the filibuster as a warning about how eliminating it could backfire when Democrats are out of power.

One common defense of the filibuster is that it’s key to ensuring that the spirit of compromise remains in the Senate. Most of the political scientists I spoke with were skeptical, though, pointing out that the supermajority requirement to end debate hasn’t always been part of the Senate rules, and has been changed several times over the course of the past century.

But the filibuster has another benefit that could make some Senate Democrats reluctant to get rid of it — the 60-vote requirement insulates the majority party from blame when it fails to deliver on big, sweeping policies. One hypothesis I heard suggested that Senate Republicans didn’t do away with the legislative filibuster during the first two years of Trump’s presidency because it gave them an excuse to avoid acting on the more politically risky aspects of Trump’s agenda, like the border wall. It also saves middle-of-the-road senators from being responsible for the fate of controversial legislation. “If the filibuster is still around, a moderate Democrat like Joe Manchin doesn’t have to be the deciding vote on Medicare for All,” Koger said.

Liberal advocates told me they’ll cross that bridge when they come to it — for now, they want to make sure the issue continues to pick up steam in the primary. Their pitch to politicians and voters is that a policy like the Green New Deal is just too important to be defeated by worries about changing Senate procedure. “I get the concerns about the Democrats losing their voice when they’re in the minority,” said Aaron Belkin, a political science professor at San Francisco State University and progressive activist. “But you have to weigh that against the cost of not acting in response to impending planetary disaster.”

And in the long term, most of the experts I spoke with said the filibuster’s days are probably numbered. “Eventually, one of the parties is going to get control of Congress and the White House and want to actually enact the policies they ran on,” Chafetz said. “When that happens, the filibuster is toast.”