Democratic presidential candidate Hillary Clinton and Sen. Bernie Sanders greet supporters at a rally where Sanders endorsed Clinton in Portsmouth, N.H., last month. (Brian Snyder/Reuters)

The Working Families Party will endorse Hillary Clinton on Tuesday, the latest show of support from a progressive group that had worked to defeat the Democratic nominee in the primaries.

"We were pretty enthusiastic for Bernie; he told the truth, and we liked it," said Dan Cantor, the national director of the WFP, referring to Clinton's primary rival, Sen. Bernie Sanders of Vermont. "We’re now shifting, obviously. There’s a pretty important election coming up. There’s overwhelming support for Clinton. And we're going to continue the political revolution in every district we can."

The WFP, founded in New York in 1998, has grown into an independent political force with chapters in 12 states, advocating for progressive policy goals like a $15 minimum wage and pushing to elect its allies in primaries. Most of its gains have come in state legislative and municipal races, but in December 2015, 87 percent of its membership and most of its board (composed of two members from each state and some additional leaders) voted overwhelmingly to back Sanders for president.

According to Cantor, 60 percent of members were needed to make the move to Clinton, and 68 percent did so. Most of the holdouts preferred that the WFP make no endorsement; few members, he said, wanted the WFP to get behind the Green Party.

"We’re all about building progressive infrastructure," Cantor said. "You don’t do that by running another progressive candidate when there's so much on the line."

Speaking in Warren, Mich., on Aug. 11, Democratic presidential nominee Hillary Clinton outlined her vision for the U.S. economy. (The Washington Post)

The decision echoes the one that New York's WFP made in 2014, when the author and law professor Zephyr Teachout ran for governor. In New York's fusion system, Teachout and Gov. Andrew Cuomo (D-N.Y.) were both able to seek the nominations of the Democratic Party and the WFP. At its state convention, WFP members were acutely aware that Teachout was an underdog and that she would remain on the ballot, splitting votes, if she won their nomination and Cuomo was chosen again by the Democrats.

Cuomo narrowly won the WFP's support, promising to deliver on a few key issues — and promptly disappointing. That disappointment fed into the WFP's first-ever presidential endorsement. WFP leadership, like leaders of many progressive groups, now say the Sanders campaign achieved major gains that will make the Democrats more open and electable.

"We’re extremely proud and happy with what Sanders accomplished," Cantor said. "He’s endorsed Clinton for a reason: She was running to deliver on a progressive platform. We hold the same view as the senator. Some people disagree, sure, but the overwhelming majority of our members understand we can walk and chew gum at the same time. We can work to elect her, and we can hold her accountable."

That message will be shared with every supporter of the party in a statement Tuesday explaining the decision. In part, it reads that a President Clinton will "be only be as good as we — social movements, unions, progressive activists, citizens and soon-to-be citizens — make her" and that the WFP will continue to make bigger changes "than what the Democratic Party or modern capitalism" currently allow.

"History is clear on this," reads the statement. "LBJ’s achievements on civil rights and the safety net expansion were unimaginable without the civil rights movement; FDR’s New Deal would have been impossible without the mobilization of millions of unemployed and industrial workers. It's up to us to set the stage for the future we want to see."

The WFP's decision comes after some of Sanders's most prominent endorsers got behind Clinton. The Communications Workers of America, the largest pro-Sanders union, endorsed Clinton; MoveOn, which backed Sanders in January, said after the final primaries that Clinton was the Democratic nominee and deserved progressive support.

In current polling, Clinton claims support from more than 90 percent of self-identified Democrats; she does slightly worse among Sanders supporters, many of whom identified as independents.

At a rally for Democratic presidential nominee Hillary Clinton, Aug. 15, Vice President Joe Biden talked about his personal connections to Scranton, Pa., to make the case that Clinton understands people with "grit and courage." (Sarah Parnass/The Washington Post)