Bernie Sanders said Tuesday that it would be "irresponsible" for his loyalists not to support Joe Biden, warning that progressives who "sit on their hands" in the months ahead would simply enable President Donald Trump's reelection.

And lest there be any question, the 78-year-old Vermont senator confirmed that "it's probably a very fair assumption" that he would not run for president again. He added, with a laugh: "One can't predict the future."

Sanders, who suspended his presidential bid last week, spoke at length about his decision to endorse Biden, his political future and the urgent need to unify the Democratic Party during an interview with The Associated Press. He railed against the Republican president but also offered pointed criticism at his own supporters who have so far resisted his vow to do whatever it takes to help Biden win the presidency.

He seemed to distance himself from his campaign's former national press secretary, Briahna Joy Gray, when asked about her recent statement on social media refusing to endorse Biden.

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"She is my former press secretary — not on the payroll," Sanders noted. A spokesman later clarified that all campaign staffers were no longer on the payroll as of Tuesday, though they will get a severance check in May.

Sanders said his supporters have a simple choice now that Biden has emerged as the presumptive nominee: "Do we be as active as we can in electing Joe Biden and doing everything we can to move Joe and his campaign in a more progressive direction? Or do we choose to sit it out and allow the most dangerous president in modern American history to get reelected?"

He continued: "I believe that it's irresponsible for anybody to say, 'Well, I disagree with Joe Biden — I disagree with Joe Biden! — and therefore I'm not going to be involved.'"

Sanders said he would not actively campaign or spend money on advertising in the primary contests that are still on the calendar in the coming months. But he still encouraged Democrats in those states to vote for him, hoping to amass as many delegates as possible for leverage to shape the party platform and the direction of Biden's campaign.

He also vowed to continue fighting for progressive priorities such as his signature "Medicare for All" as a senator, even though Biden has refused to embrace the government-backed single-payer health care system.

"If people want to vote for me, we'd appreciate it," Sanders said of the roughly 20 primary contests that remain where his name will appear on the ballot. He later added, "I think you're going to see significant movement on the part of the Biden campaign into a more progressive direction on a whole lot of issues."

Sanders did not outline any specific plans to begin helping Biden in earnest, though he noted that he held dozens of rallies for former Democratic nominee Hillary Clinton four years ago and would be at least as active for Biden. In the short term, he said he's essentially "incarcerated in his home" because of coronavirus social distancing guidelines and did not know when he would return to the campaign trail.

Sanders brushed away questions about why he was willing to back Biden so much sooner than he did Clinton, whom he waited until June to endorse. He said recent conversations with former President Barack Obama, who endorsed Biden on Tuesday, did not influence his decision. It came down to simple math, he said.

In 2016, Sanders said he had a mathematical path to the nomination all the way until the California primary, which was held on the last day of voting in June. That simply wasn't the case this year.

"What would be the sense of staying in, of spending a whole lot of money, of attacking the vice president, giving fodder for Trump — what's the sense of doing that when you can't win?" he asked.

"I will do everything I can to help elect Joe," Sanders continued. "We had a contentious campaign. We disagree on issues. But my job now is to not only rally my supporters, but to do everything I can to bring the party together to see that (Trump) is not elected president."

While Sanders is urging his backers to throw their weight behind Biden, some of his top surrogates have so far not endorsed him. Congresswoman Alexandria Ocasio-Cortez, a Democrat from New York, told the New York Times this week she would support the Democratic presidential nominee in November, but stopped short of endorsing Biden.

Ocasio-Cortez also said Tuesday night "it's legitimate" to discuss a sexual assault allegation leveled against Biden by Tara Reade, who worked for him when he was a senator.

"If we want, if we again want to have integrity, you can't say, you know — both believe women, support all of this, until it inconveniences you, until it inconveniences us," Ocasio-Cortez said.

Biden's deputy campaign manager, Kate Bedingfield, said the allegation against Biden was "untrue."