In her 2000 and 2006 campaigns for the US Senate, Hillary Clinton ran not just on the ballot line of the Democratic Party but also on the ballot line of New York’s Working Families Party.

Backed by unions and grassroots activists, the WFP was organized in 1998 to move New York and national politics in a progressive direction. Sometimes, when the fight has been with conservative Republicans, the WFP has aligned with centrist Democrats, including Clinton. But, when the fight is over the type of challenge that will be made to those conservative Republicans in 2016, WFP activists recognize that progressives are skeptical with regard to Clinton’s expected candidacy for the Democratic presidential nomination.

A measure of that skepticism was on display Sunday, as the Working Families Party formally urged Massachusetts Senator Elizabeth Warren to try for the Democratic nod.

“We know a champion for working families when we see one—and millions of Americans clearly do too. That’s why Senator Warren is inspiring such a remarkable surge of grassroots enthusiasm,” declared a message from WFP New York State Director Bill Lipton, which was sent to party backers and others after the WFP’s Sunday decision to ask Warren to run. “She’s taken the fight directly to Wall St. and to the big banks that increasingly have a vice grip on our economy and our democracy. She’s led the effort to create a Consumer Financial Protection Bureau. She’s fought on behalf of students and recent graduates suffering from crippling student loan debt, and to change the debate in D.C. from a discussion over whether to cut Social Security into one about how we can grow it.”

Warren says she is not running.

But an energetic “Draft Warren” campaign has taken shape in Iowa, New Hampshire and other states. (Read more about it in my colleague George Zornick’s Nation cover story). National groups such as MoveOn.org and Democracy for America are pouring energy into the effort. And the WFP move turns up the volume.

A significant force in New York City, where Mayor Bill de Blasio and Public Advocate Letitia James have long enjoyed the party’s support, the WFP maintained its ballot line in 2014 by backing New York Governor Andrew Cuomo. The Cuomo endorsement stirred criticism from progressive activists, who thought the party should challenge a governor who was seen as being too close to the interests the WFP frequently battles. The call for a Warren candidacy puts the WFP at odds with a candidate it once endorsed, Clinton, but it is likely to appeal to the progressives activists who last year were frustrated with the party.

The move also marks the party as a potentially influential force in presidential politics. Operating in a state where third, fourth and even fifth parties can be significant political forces—thanks to a “fusion” law that allows candidates to run on more than one party line and then merge accumulated votes into a grand total—the Working Families Party is not the first of New York State’s smaller parties to link up not merely with major parties but with movements within major parties. When John F. Kennedy accepted the endorsement of the state’s old Liberal Party in 1960, he declared, “I’m proud to say I’m a Liberal.” Kennedy’s Democratic vote in New York state that November was slightly less than Richard Nixon’s Republican vote, but 406,176 Liberal votes gave Kennedy a comfortable victory.

The prospect of combining Democratic and WFP votes for Warren is a long way off. A Warren spokesperson told The New York Times Sunday that the senator “is not running for president and doesn’t support these draft campaigns.” And Warren is far behind Clinton is the polls.

That said, leaders of the movement to draft Warren were clearly pleased with the move by the WFP.

Referring to the WFP decision as a “a huge moment for the campaign to draft Elizabeth Warren,” Ready for Warren campaign manager Erica Sagrans said, “Last summer, a group of passionate volunteers who shared the improbable idea that they could convince Elizabeth Warren to run for president came together to start Ready for Warren. Late last year, MoveOn and Democracy for America joined in. And now with the Working Families Party on board, this movement is getting stronger by the day.”