ATLANTA (AP) -- When Tom Perez stepped to the stage as the newly elected Democratic national chairman, his first official act was to invite his vanquished rival to join him as deputy chairman.

Minnesota Rep. Keith Ellison accepted on the spot and two men stood together, smiling like a national ticket at a presidential nominating convention.

Members of the Democratic National Committee cheered wildly at their gathering in Atlanta on Saturday, forgetting the competitive race that took two rounds of voting _ unprecedented in recent memory for either major party. They picked Perez, a former labor secretary backed by former President Barack Obama, over Ellison, backed by liberal Sen. Bernie Sanders of Vermont.

Yet piercing the cheers were boos, yells and expletives from more than a few young Ellison supporters in the gallery, some of them in tears. Reaction wasn't enthusiastic among the liberal groups that had embraced Sanders' presidential campaign and have intensified their efforts since the election of Republican President Donald Trump.

"We don't have the luxury of walking out of this room divided," Ellison said over the jeers. Afterward, he told reporters he trusts Perez and that the burgeoning resistance movement aimed at President Donald Trump should do the same.

The reaction Saturday -- and the unusual campaign that preceded it -- underscores the challenges Perez and Democrats face as they look to recover from a disastrous electoral slide that was obscured by Obama's two national victories but laid bare by Hillary Clinton's stinging loss to Trump.

Now, besides Trump occupying the Oval Office, Republicans control Congress and about two-thirds of statehouses, and they're one Senate confirmation vote away from a conservative majority on the Supreme Court.

"We suffer from a crisis of confidence ... a crisis of relevance," Perez told DNC members ahead of his election.

Trump took to his preferred medium to rub it in. "Congratulations to Thomas Perez, who has just been named Chairman of the DNC. I could not be happier for him, or for the Republican Party!" the president wrote on Twitter. Early Sunday, the president asserted that the Democratic contest was "of course, totally `rigged.' Bernie's guy, like Bernie himself, never had a chance. Clinton demanded Perez!"

Perez and Ellison have agreed to capitalize on the widespread opposition to Trump while rebuilding moribund state and local party organizations.

"We are all in this together," Perez said. Repeating a line he's used for weeks while campaigning for the post, he praised public demonstrations against Trump as a statement from "millions of Americans" that "Donald Trump does not represent our values."

But it was Ellison who tapped so much of that anti-Trump energy. Sanders released a statement congratulating Perez yet warned "it is imperative that Tom understands that the same-old, same-old is not working."

Jim Dean, chair of Democracy for America, called Perez's election "incredibly disappointing" and said the "resistance will persist ... with or without the leadership of the Democratic National Committee." Dan Kantor, leader of the Working Families Party, said Democrats "missed an opportunity."

Jeff Weaver, who managed Sanders' presidential campaign and now runs the offshoot Our Revolution group, said Perez and Ellison must prove themselves as a leadership duo.

"Do they really invest in 50 state parties? Are they really going to engage with people in the streets?" Weaver asked. "Are they really going to bring those people into the party, so that you have a much broader cross-section of democratically aligned voters represented?"

Even a top labor leader, Lee Saunders, who worked with Perez when he ran the Labor Department, issued a statement that congratulated Ellison before mentioning Perez and praising him as a "tenacious champion for working families." Saunders leads the union of federal, state and local public employees.

The son of Dominican immigrants, Perez actually comes to the job with a demonstrably liberal record as a civil rights attorney and backer of organized labor. In the chairman's race he carried the establishment label as a Maryland resident who's spent years in the Washington orbit, working in the Justice Department and ultimately as an Obama Cabinet secretary.

Besides his support from Obama, he had the public endorsement of former Vice President Joe Biden, and one of Obama's closest aides, Valerie Jarrett, made calls to DNC members on Perez's behalf as late as Friday night. Some Clinton insiders aligned with him, as well.

Several Republican organizations seized on that Saturday, with one calling Perez "Hillary 2.0."

Perez has embraced the idea of a more aggressive, populist identity for the party, even if he hasn't convinced activists he can deliver on it. He said throughout the three-day DNC meeting ahead of the vote that he would work to align party resources with the energy of groups from Black Lives Matter and Swing Left to Indivisible, Resist Trump Tuesdays, Knock Every Door, Rise Stronger and Sister District.

He promised "a culture change" at the top echelons of the party, adding, "We have a lot of work to do."