To press the issue, Sanders veterans, along with allied activists and organizers, have launched a new political action committee called We Will Replace You. The group is demanding that Democrats on Capitol Hill uniformly oppose all Trump nominees, including Supreme Court nominee Neil Gorsuch, demand the firing of top Trump strategist Steve Bannon and use all the levers of their limited congressional power to gum up the White House agenda -- or face opposition from within their own party.

They are also asking new supporters to sign a pledge -- written at the top of their homepage -- promising to back "primary election challengers against any Democrats who won't do everything in their power to resist Trump."

"Democrats need to know there is an actual political cost and this isn't just going to be folks showing up at their offices, but folks showing up at the ballot and different organizations supporting challengers who are going to push the party in a different direction," said Max Berger, a co-founder of #AllOfUs, the millennial progressive group that launched the new campaign.

Early opposition to the Trump administration, most visibly in the form of mass protests and rowdy recriminations against Republicans at town hall meetings around the country, has turned up the heat on long-simmering efforts by the left to pressure moderate Democrats. With the party now totally out of power in Washington and at a crossroads, activists who gained experience during Occupy Wall Street and through work with the Movement for Black Lives, the Fight for $15 and other aligned causes see an opportunity for greater influence.

"We've had a generation of protests where people have learned how to fight those in power. But eventually, you get to a point where you realize that it's necessary for the communities that you represent to actually have power and not just to protest," Berger said. "The leaders that we see coming out of those movements are now looking to win elections and represent the communities they have been serving for the past decade."

We Will Replace You is operating as a hybrid PAC, meaning it can raise money and offer capped support to specific candidates while also making independent expenditures from a separate account. Co-founder Claire Sandberg, a former digital organizing director for the Sanders campaign, said the group is banking on a financial groundswell, delivered through ActBlue and other familiar channels, to deliver an early boost.

"We've seen the power of what an army of small dollar donors and grassroots volunteers can do when they are asked to do something that they believe in," she said. "We don't think that we need a giant pile of cash to make this project extremely successful electorally."

As Republicans learned earlier this decade, dedicated efforts to influence policy from within by launching contentious primary fights can yield mixed results. For every Mike Lee or Ted Cruz, both tea party-backed candidates who took on the GOP establishment before knocking off Democratic opponents in Senate races, there have been cautionary tales, like Sharron Angle and Richard Mourdock, who fumbled away seats Republicans expected to win.

Democrats have little margin for error in 2018, when 10 of their own come up for re-election. Republicans currently hold 52 seats in the upper chamber. If the GOP can flip eight more, they will claim a filibuster-proof majority and go forward with virtually no constraints on their legislative agenda.

Sandberg dismissed concerns, most often voiced by party centrists who backed Hillary Clinton in the 2016 primary, that a "tea party of the left" could harm Democrats on Election Day.

"We reject out of hand the notion that pushing Democrats to be better candidates will lead to more Republican victories," she said. "The much greater danger is a Democratic base that is uninspired by the party's tepid response to the Trump administration will not feel motivated to turn out."

We Will Replace You expects to ramp up its efforts in the summer. It has not yet named or set its sights on any particular race, though it could offer support to Virginia gubernatiorial hopeful Tom Perriello, who is running this year in a primary many Democrats will look at as a bellwether for 2018.

Adam Green, co-founder of the Progressive Change Campaign Committee, told CNN that while there has been "a constant appetite" for pitting progressive newcomers against establishment picks in open seat primaries, the increased pressure on elected Democrats has been a long time coming.

"There's been ebbs and flows in the willingness to primary incumbents and that will likely be way more on the table in 2018 than it's been in past cycles," he said. "And most likely there will be at least one clear poster child that people identify and collaborate around."

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