Mr. Biden, speaking to donors via videoconference at a fund-raiser on Wednesday, called Mr. Sanders “a powerful voice for a fairer and more just America” and previewed the incorporation of some elements of the Sanders agenda into his own. “I’m committed to seeing that these good ideas, wherever I can find them on every issue, are brought into the campaign,” he said.

The lowering of the Medicare age to 60 from 65 is a small step compared with the universal, government-run “Medicare for all” plan that Mr. Sanders has championed. But it is a symbolically significant one, given that Mr. Sanders has made expanding access to health care a centerpiece of his platform.

Mr. Biden’s student debt forgiveness plan — which would eliminate student debt for low-income and middle-class people who attended public colleges and universities, historically black colleges and universities, and other institutions that serve students of color — also does not go as far as Mr. Sanders’s plan to cancel all student debt.

Peter Hart, a Democratic pollster, said he was less concerned about party unity now than in 2016, when some disaffected Democrats who were aligned with Mr. Sanders in the primary race sat out the general election rather than vote for Hillary Clinton.

“The galvanizing force in 2020 is Donald Trump,” Mr. Hart said. Unity is “not a problem,” he added, “because the hatred and the fear of Donald Trump is more important than any single issue position which may divide the Sanders forces and the Biden forces.”

One concern among some Biden-aligned Democrats is not just what Mr. Sanders himself says, but whether the senator is willing to publicly disagree with some of his more prominent supporters and amplifiers in the progressive ecosystem who publicly undercut Mr. Biden.

Close to 2 a.m. on Thursday, Mr. Sanders’s national press secretary, Briahna Joy Gray, linked on Twitter to a segment on Fox News that sought to raise questions about Mr. Biden’s mental acuity. “Bernie was too kind to go after Biden, but it’s coming,” she warned. “Either Dem leadership cares more abt maintaining a corporate status quo than getting rid of Trump, or they’re planning to replace Joe.”