Sen. Bernie Sanders (I-Vt.) made a motion to suspend the rules and declare Hillary Clinton the official Democratic presidential nominee. (Video: The Washington Post, Photo: Ricky Carioti/The Washington Post)

In parks, on trains and outside Philadelphia City Hall, the "Bernie or Bust" movement was having its biggest day ever. Some participated in a mock roll-call vote to reflect how they wanted the Democratic primary to go. Some slapped the sunflower logo of the Green Party over fading signs for Bernie Sanders.

In quieter and more secluded rooms, Sanders — the unwilling hero of liberals unwilling to support Hillary Clinton for president — was negotiating the terms of her nomination.

On Tuesday evening, Sanders closed the roll call of states’ votes by supporting Clinton and trying to bring the entire Democratic National Convention around to her. It was the latest necessary step if Sanders was to do what few defeated insurgents have done: transform a presidential bid into a coherent and lasting movement of new voters.

“They had never been in a Democratic Party meeting,” Sanders said at a Tuesday morning breakfast with reporters. “They wanted to get involved. And it would be a terrible, terrible shame if we do not figure out a way to capture that energy, to capture that idealism, to capture that love of this country.”

As he has wound down his presidential campaign, Sanders has begun to build new organizations and plan for a possible power move if Democrats regain the Senate. He has officially launched Our Revolution, a 501(c)(4) “social welfare” group to build support for liberal policies.

It’s the first of a possible trio of Sanders projects. Another will seek to elect like-minded candidates up and down the ballot across the country, and a third possible group might engage in other political activity.

Sanders is not the first defeated candidate to nudge supporters toward a new political venture. Howard Dean’s 2004 campaign gave birth to Democracy for America, which has funded and promoted progressive candidates from Congress to the presidency. Progressive Democrats for America, founded in the wake of former Ohio congressman Dennis Kucinich’s gadfly 2004 bid, backed Sanders early in 2015 — and is part of a scattershot effort to protest the Clinton-Timothy M. Kaine nomination.

More than those candidates, Sanders has the potential to build power in the Capitol if Democrats regain the White House. Sanders has been angling to become chairman of the Committee on Health, Education, Labor and Pension, on which he is the second-ranking member of the Democratic caucus, behind Sen. Patty Murray of Washington. If Murray opts to chair another committee, Sanders would be in line to get the gavel.

If that happens, “he could do a lot of the things he’s talked about,” said Sanders spokesman Michael Briggs.

The HELP committee has jurisdiction over several initiatives Sanders has pushed on the campaign trail and plans to pursue in the Senate, including raising the federal minimum wage to $15 an hour, making tuition at public colleges and universities free, and expanding access to government-run health insurance.

Briggs acknowledged that it’s in Sanders’s best interest as a senator to have Clinton in the White House. But, he said, “it’s also in America’s interests.”

In the weeks since Clinton clinched the nomination, Sanders has successfully pushed her campaign to adopt some of his policy proposals, including some related to college affordability and access to health care. The two camps have grown even closer, out of necessity, as they’ve strategized ways to limit possible acrimony at the convention.

Larry Sanders, the older brother of Sen. Bernie Sanders (I-Vt.), cast his vote for his brother during the second day of the Democratic National Convention. (Video: Victoria Walker/The Washington Post)

Aides to Sanders are hopeful that the relationship between the two camps is something that could be built upon with Clinton in the White House. According to Briggs, Sanders is committed to turning the 2016 party platform — which he repeatedly calls “the most progressive” in the party’s two centuries of existence — into reality in the Senate. Without a Democratic Senate and Clinton in the White House, those efforts are certain to be non-starters.

Unlike Clinton after her 2008 loss to Barack Obama, Sanders is not seen as a possible member of the next presidential Cabinet. “Honestly, I have not thought about it,” Sanders said at the breakfast, which was sponsored by Bloomberg Politics. “I love my state and the people of my state.”

The idea of Sanders growing even closer to Clinton is anathema to some Sanders supporters, who made that feeling known at Tuesday’s delegation meetings. For an hour, many Sanders delegates worried that a “roll call” scheduled for the breakfasts would replace the televised afternoon roll call where they could vote for their candidate.

That added tension to an ongoing negotiation between all levels of the campaigns, including Sanders delegates who have been working the floors to calm delegates and prevent protests. Clinton’s allies responded by hugging Sanders even tighter. When the candidate arrived at the New York delegation’s breakfast, he shared a stage with Gov. Andrew Cuomo (D-N.Y.), a villain for many progressives who surprised them by praising Sanders’s advocacy for a $15 minimum wage.

“You can take the boy out of Brooklyn, but you can’t take Brooklyn out of the boy,” Cuomo said.

While Sanders is singing Clinton’s praises now, he was critical of her in several respects during the primaries. The Democratic contest never got as nasty as on the Republican side, but at several points Sanders questioned Clinton’s judgment to be president, including for her support of the Iraq War.

He also mocked her for accepting speaking fees from Goldman Sachs and refusing to release the transcripts and repeatedly suggested that Clinton, a former senator from New York, is too close to Wall Street interests.

On the way into Tuesday night’s sessions, a Sanders delegate, Daraka Larimore-Hall, shared a train car with Clinton delegate Reggie Jones-Sawyer. Both were from Southern California. Both politely ignored a hubbub from “Bernie or Bust” protesters as they discussed how the party could come together.

“I’m going to wrap Trump around every Republican candidate, all the way down to school board,” Larimore-Hall said.

“They’re here to talk about free college education — I wish we had that when my kids were in school,” said Jones-Sawyer, a California state assemblyman. “But I’ve been through the Deaniacs, the Kerryacs, Obama for America. They were loud, they were proud. Eight years later, poof. I want to see how they sustain it.”