Add Beto O’Rourke to a growing list of Democratic presidential candidates who are considering scrapping long-standing Senate procedure in hopes of passing a sweeping progressive agenda should they make it to the White House.

Under siege is the filibuster, the longstanding Senate tradition requiring 60 votes in the 100-member chamber to advance a bill, effectively allowing the minority party to block legislation.

WHAT IS A FILIBUSTER?

“I think that that’s something that we should seriously consider,” O’Rourke told reporters on the campaign trail in New Hampshire earlier this week.

“We have to look at some of these institutional reforms, whether it’s the Supreme Court, the Electoral College, the filibuster in the Senate, we’ve got to get democracy and our institutions working again,” explained the former three-term congressman from Texas.

On the same day that O’Rourke entertained the idea, a rival for the Democratic nomination also opened the door to the idea of dispatching with the filibuster.

“When you talk about changing the filibuster rule I understand that we are heading, right now, we are heading that way,” Sen. Cory Booker of New Jersey said in an interview on "Pod Save America." “I’m going to tell you that for me that door is not closed.”

The comments mark an increasing appetite in the 2020 Democratic field for challenging longstanding political traditions and institutions -- everything from the voting age to the Electoral College to the Senate filibuster. And for Booker, his comments mark a backtrack from previous statements.

Last month, Booker told NPR that he didn’t favor eliminating the filibuster. And in an interview with Politico in January – before he formally declared his candidacy – he said “we should not be doing anything to mess with the strength of the filibuster. It’s one of the distinguishing factors of this body. And I think it is good to have the power of the filibuster.”

The pro-Republican opposition research shop America Rising accused the senator of flip-flopping on the issue, saying in an email after the senator’s latest comments that “Booker has jumped on board with the latest liberal litmus test, abolishing the filibuster.”

TRUMP CALLS FOR SCRAPPING FILIBUSTER TO BUILD WALL

At the moment, the filibuster is actually helping the Democratic Party, enabling its members to slow or stall legislation that the GOP Senate majority and Trump White House might support. Trump himself has called for an end to the filibuster, only to be met with opposition from Senate Majority Leader Mitch McConnell.

But McConnell lowered the threshold to confirm Supreme Court nominees to a simple majority, and other federal judges and Cabinet nominees also are no longer subject to a 60-vote threshold. The filibuster on legislation is all that remains in terms of built-in brakes in the upper chamber that could slow the majority party.

And so Democrats hoping to pass a sweeping progressive agenda if they win back the White House are concerned their proposals could get bottlenecked in the Senate, where the Democrats have a shot at winning back control -- but have little chance of grabbing a 60-member, filibuster-proof majority.

“Everything stays on the table. You keep it all on the table. Don’t take anything off the table,” Sen. Elizabeth Warren said when recently asked on the presidential campaign trail about scrapping the filibuster.

Candidates proposing major changes to deal with climate change also see the filibuster as a major impediment.

"I don't believe you can be serious about saying you can defeat climate change unless you realize we need to have the filibuster go the way of history because Mitch McConnell has weaponized the filibuster," Washington State Gov. Jay Inslee told reporters on Wednesday. "You can't be serious about having major decarbonization legislation in any near-term without removing the filibuster."

But not all of the White House contenders are on board.

“Great question…Let’s change the subject!” joked Sen. Kamala Harris of California, when asked by a voter in Iowa about her stance on the issue.

The Harris campaign tells Fox News that their candidate has “said she's genuinely conflicted on this issue but everything is on the table.”

Sen. Kirsten Gillibrand of New York told "Pod Save America" in January that “I think it’s useful to bring people together, and I don’t mind that you have to get 60 votes for cloture.”

“If you’re not able to get 60 votes on something, it just means you haven’t worked hard enough, talking to enough people and trying to listen to their concerns and then coming up with a solution that they can support. And so I’m not afraid of it one way or the other,” she added.

Sen. Bernie Sanders also opposes scrapping the filibuster.

"I'm not crazy about getting rid of the filibuster. I think the problem is, people often talk about the lack of comity and the anger. The real issue is that you have in Washington a system which is dominated in Washington by wealthy campaign contributors,” he said last month in an interview with CBS News.