AFL-CIO President Richard Trumka on Tuesday criticized Democratic presidential frontrunner Hillary Clinton for not doing enough to "energize workers" -- amid speculation over whether big labor might back an alternative candidate like Vice President Biden. 

Trumka declined to endorse anybody and suggested his group would stay on the sidelines for now. But he addressed the 2016 contest just days after holding a private meeting with Biden, who is weighing entering the race. 

“The vice president is a good friend and a great champion for working people," Trumka said, at a Washington forum hosted by The Christian Science Monitor. But he added he doesn't know whether Biden will run.

The notion of a third Biden presidential run has caught fire in recent weeks, amid concern in the Democratic Party about Clinton’s declining poll numbers. Biden fueled the speculation by meeting recently not only with Trumka but Massachusetts Democratic Sen. Elizabeth Warren, whose anti-Wall Street message has generated a loyal liberal following. 

“He would make a good candidate and a good president,” Trumka said of Biden.

But the union boss didn't dismiss Clinton, either. He called the former senator and secretary of state “an experienced person” who would make a “great president.”

“But she’s got to find a way to energize workers,” he said. “Any candidate is going to have to be active on increasing wages.”

Trumka said that Clinton “has to do A-plus work to get a C” because she’s a woman.

Trumka, meanwhile, said the union’s goals in 2016 are the same as in previous election cycles -- to get candidates to articulate their vision for a “raising-wages” agenda, then “hold them to it.”

“That’s still the yardstick,” he told reporters. 

Trumka said an endorsement before Iowa’s first-in-the-country caucuses in February is “not likely, though conceivable” because AFL-CIO leaders are still gathering input for members across the country. And he made clear he doesn’t pick a candidate, and only announces the will of the members.

Membership in labor unions has declined in recent decades as the U.S. economy shed manufacturing jobs. But they remain influential in Democratic Party politics, in large part because of their voter-organizing and -outreach efforts. 

Trumka also praised 2016 Democratic candidate Vermont Sen. Bernie Sanders, who is closing on Clinton’s lead in Iowa and elsewhere and drawing large crowds on the trail. 

“He has a very unique and genuine way of connecting with voters,” Trumka said.