Vice President Joe Biden meets with students who participate in Step IT Up, an accelerated learning program which trains workers for high-growth and well paying IT jobs in Detroit. (Carlos Osorio/AP)

Vice President Biden delivered a full-throated call to action for progressive activists here Thursday, burnishing his credentials with an important segment of the Democratic base that could pay dividends if he runs for president in 2016.

Even before taking the stage at Netroots Nation, an annual gathering of several thousand liberal activists, Biden was celebrated as a moral hero to the movement.

“Joe, we’ve been watching you,” said Arshad Hasan, executive director of ProgressNow, as he introduced the vice president. “The history books will write down that the first sitting president to endorse marriage equality was our president, Barack Obama. . . . But those of us who hold marriage equality near and dear to our hearts and our homes, we know that Joe Biden spoke first.”

Hasan’s introduction was a powerful validation of Biden’s liberal bona fides, and the vice president reveled in his status as a leading voice for gay rights.

“I come out of the civil rights movement,” Biden said. Referring to his surprise comments in 2012 declaring support of same-sex marriage, Biden added: “There’s no way [on] God’s green Earth that I could sit there and be asked a question about the civil-rights issue of our day and remain silent. It would’ve made a lie of who I am.”

Same-sex marriage status in the U.S., state-by-state

Biden’s endorsement of gay marriage, in an interview on NBC’s “Meet the Press,” foiled plans by President Obama’s political team for the president to support same-sex marriage later in the reelection year.

In his speech Thursday, Biden said there was a “tactical difference” at the White House but that he felt “enough is enough” and it was time to speak his mind.

“I make no apologies — none,” Biden said.

Biden’s visit to Netroots is the latest in a series of speeches he is delivering to key Democratic constituency groups this summer. On Wednesday, he addressed hundreds of millennials in Washington at Generation Progress, a youth gathering sponsored by the Center for American Progress. Next week, he will speak at the NAACP’s annual convention in Las Vegas.

Biden’s appearances are designed to engage the Democratic faithful heading into a tough midterm campaign season, when the party’s control of the Senate is at serious risk. But the visits also afford the vice president a chance to showcase his commitment to liberal policies as he weighs whether to run for president again in 2016.

“I don’t take a back seat to anyone when it comes to fighting some of the toughest progressive battles this country’s seen,” Biden said Thursday.

He ticked through his decades of work, first in the Senate and now as vice president, on issues such as reproductive rights for women, tougher gun laws and blocking conservative Supreme Court nominees.

Biden received particularly enthusiastic applause from the audience of more than 1,000 activists when he talked about gay rights and women’s rights.

“No one should ever have a right to dismiss somebody because of their sexual orientation,” Biden said. “It just should not exist.”

At one point, Biden was interrupted by a handful of protesters who shouted, “Stop deporting our families!”

Obama has been criticized by pro-immigrant advocates for a heavy pace of deportations during his tenure and is embroiled in a debate over how to cope with an influx of unaccompanied minors over the border.

But here in Detroit, Biden paused for the protesters and said, “We should clap for those young people.”

“Can you imagine the pain, the anxiety of coming home every day wondering whether or not your mother or father would still be there?” he asked.

Biden also talked about efforts by Republicans in many states to change laws to make it more difficult for citizens to vote.

“This is a fight that I know is not [at] the top of everyone’s agenda, but we cannot lose,” Biden said. “These guys mean it. They haven’t gone away.”

Sen. Elizabeth Warren (D-Mass.), whose message of economic populism has made her a liberal heroine, will address Netroots Nation on Friday.

Former secretary of state Hillary Rodham Clinton, who is considered an overwhelming favorite for the Democratic presidential nomination should she seek it, declined an invitation to attend the conference.

But Ready for Hillary, the pro-Clinton super PAC, was a sponsor of the conference and has representatives here organizing on her behalf.