Most of the Democrats running for president want to create a national single-payer health care system. They want to begin a massive transition from fossil fuels to clean energy. They want to legalize marijuana, pass broad family leave policies, and do a whole lot of other things that previous generations of presidential candidates have balked from fully endorsing.

But most of these presidential candidates are shying away from endorsing — or outright opposing — a Senate rules change that a growing number of progressive activists say would be essential to making any of these proposals reality.

They're calling on Democratic candidates to endorse ending the legislative filibuster, which requires support from at least 60 senators — in almost all cases, that means bipartisan support — to pass most bills. "In order to actually pass a big, bold pro-democracy package, or a big, bold climate package, or a health care package, we're going to need to be able to do that with 51 votes," said Ezra Levin, the co-founder of the grassroots organizing group the Indivisible Project.

The push to eliminate a rule that isn't constitutional, but is so bound into Senate history and culture that it may as well be, has this field of unapologetic progressives uncharacteristically cautious.

"Great question," California Sen. Kamala Harris, a candidate running on a promise to "speak truths," said when an Iowa voter asked her about killing the legislative filibuster. "Let's change the subject!"

Harris isn't the only candidate cautious about the idea. Many senators see the filibuster as the one essential tool a minority party has to hold leverage and block legislation it opposes. Fresh off a two-year stretch where Republicans held all the levers of power in Washington, D.C., many Democratic senators see the hassles of the filibuster when a party is in the majority as worth the cost of the protections it affords for years in the minority.

A brief history of the filibuster for those who aren't steeped in Senate history: it's not in the U.S. Constitution, but for most of the Senate's history, the body has required a super-majority of members to agree to end debate and move forward to final votes. That meant even if the majority of lawmakers supported a bill, or a judicial nominee, a large-enough minority could block a vote.

In the face of increased Republican filibustering, Majority Leader Harry Reid, a Democrat, eliminated the filibuster for lower court and executive nominations in 2013. Four years later, when Republicans held the Senate and the White House, Senate Majority Leader Mitch McConnell, R-Ky., eliminated filibusters for Supreme Court picks. That meant all these nominees could be confirmed with simple majorities, rather than needing 60 votes of support. Supreme Court Justices Neil Gorsuch and Brett Kavanaugh were both confirmed with fewer than 60 votes, among many other lower court nominees.

The filibuster's last stand: legislation. Most bills still need 60 votes to move forward. That's why Levin and many other progressive activists are arguing for Democrats to kill it, should the party retake the Senate in 2020.

"We know that the Republicans will utilize every tool available to them to prevent these kinds of big reforms from getting done. They've done it before. They did it during the entirety of the Obama administration," he said.

But of the seven U.S. senators running or considering running for president, only one – Massachusetts Sen. Elizabeth Warren – is even open to the idea of eliminating the barrier.

New York Sen. Kirsten Gillibrand rejected the argument that a 60 vote threshold would block all major Democratic policies during a recent interview on Pod Save America. "If you're not able to get 60 votes on something, it just means you haven't worked hard enough, talking to enough people."

Progressives like Levin point to the fact that the Obama Administration worked hard to court Republicans during the push for the Affordable Care Act and other major bills, and often got, at most, one or two GOP votes.

Still, Gillibrand's not alone. Vermont Sen. Bernie Sanders told CBS he's "not crazy about getting rid of the filibuster." California Sen. Kamala Harris is "conflicted," and said "I see arguments on both sides."

New Jersey Sen. Cory Booker is more forceful. "I will personally resist efforts to get rid of it," he told reporters on the first day of his presidential campaign.

Ohio Sen. Sherrod Brown and Minnesota Sen. Amy Klobuchar both signed a letter urging the Senate to keep the legislative filibuster in 2017, when President Trump began his on-again, off-again Twitter calls to blow up the rule.

But Warren indicated it's a proposal worth considering. "If the Republicans are going to try to block us on key pieces that we're trying to move forward, then you better believe we've got to keep all the options on the table," she told Pod Save America.

It's a sharp break from some of the non-senators in the race, including Washington Democratic Governor Jay Inslee. "The filibuster will essentially doom us to a situation where we'll never be able to fight climate change," he said, blaming the senators' support for the rule as a byproduct of their life in a fiefdom.

Delaware Sen. Chris Coons said it's not about Senate power — it's about giving minority parties protection and encouraging bipartisan.

"It's a terrible idea," Coons — who isn't running for president but has long defended the filibuster — told NPR. "Democrats would reap the whirlwind almost immediately."

"It is the last bulwark of the rights of the minority in the Senate," he said. "If a simple majority could carry the day, we'd have a right-to-work law signed into law by President Trump — I could give you a list of 20 things that President Trump would have signed into law in his first few weeks that would either repeal any positive thing that President Obama did, or anything that a progressive Democrat would hope to do in the future."

In addition to measure curbing collective bargaining powers, Democrats were able to use the filibuster last session to block attempts to repeal Obama-era financial reforms and abortion restrictions, among other measures.

Levin says he's willing to make that trade-off. "Look, Democracy is the theory that people know what they want, and they deserve to get it. And I don't think as pro-democracy progressive we can afford to be scared of the will of the people," he said.

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