WASHINGTON — The progressive advocacy group Stand Up America is putting pressure on the last two major Democratic candidates, Joe Biden and Bernie Sanders, to call for eliminating the 60-vote threshold to pass legislation in the Senate.

Ahead of the Democratic debate Sunday in Phoenix, the group plans to mobilize the 2 million members it boasts — via social media, email and a text message list of 400,000 — to press the moderators to ask the candidates about the filibuster.

"We must end the filibuster if we want to get a progressive agenda through," Sean Eldridge, the founder and president of Stand Up America, said in an interview. "We've seen Republicans time and time again block progress on issues like gun control, immigration reform, health care — and I expect we are going to continue to see more of the same."

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The effort comes as activists grow nervous that the 60-vote threshold will allow Republicans to block progressive legislation even if Democrats win control of the White House and both chambers of Congress in November. It comes after Sen. Elizabeth Warren, D-Mass., a vocal supporter of eliminating the filibuster, ended her campaign after falling short in the early states and Super Tuesday.

The push to nuke the filibuster is opposed by Biden, who was a senator for three decades, and Sanders, who has served in the chamber since 2007. It takes 50 votes in the Senate to eliminate the rule, and activists believe that would require an aggressive push from the presidential nominee.

Eldridge said the goal is to push the nominee to endorse an end to the 60-vote rule.

Under President Barack Obama, bills like the DREAM Act and near-universal background checks for gun buyers had majority support in the Senate but were thwarted by the filibuster. The former president lamented after he left office that the 60-vote rule made it "almost impossible" to govern.

Other progressive groups, like Indivisible, have also made it a priority to get rid of the filibuster. They argue that while reducing the threshold to 51 votes would also enable Republicans to pass more conservative legislation, progressives would benefit more from it.

"The best way to know that ending the filibuster would net-benefit the Democrats is that [Senate Republican leader] Mitch McConnell loves it," Indivisible co-founder Ezra Levin said.