Mr. Sanders has also made it clear that he is aware he cannot continue to campaign indefinitely and defiantly, as he did against Hillary Clinton in 2016, according to former aides and people currently in his orbit.

And for all of the criticism he has leveled at Mr. Biden recently, Mr. Sanders feels far more positively in a personal way toward Mr. Biden than he did toward Mrs. Clinton, which could make a reconciliation less contentious, according to a veteran of the last campaign. This person and others briefed on Mr. Sanders’s campaign plans insisted on anonymity in order to discuss internal deliberations.

Over the past several months, Jeff Weaver, a senior adviser to Mr. Sanders, has opened up a line of communication with one of Mr. Biden’s top political aides, Anita Dunn. Although the back-channel setup has mostly been used to arrange logistics — they hashed out an agreement this week to bar a live audience from Sunday’s debate — it is likely to be the main conduit for talks about political matters, according to people on both campaigns.

With over half the delegates still to be allocated, aides to Mr. Sanders said they saw more fertile terrain in the coming weeks. They point out that Mr. Sanders narrowly lost Illinois four years ago and see strength in Wisconsin and Puerto Rico. Georgia, with its heavily black electorate, will most likely go to Mr. Biden, who currently leads Mr. Sanders in delegates, 864 to 710, with delegates still to be counted.

Mr. Sanders also has about $9 million worth of ads booked through March 17, including a $2 million buy this week.

Aides say Mr. Sanders is particularly motivated to participate in the debate on Sunday in Phoenix because he views it as his first and possibly last opportunity to face Mr. Biden one on one and hold him accountable for his record, while trumpeting his own record in contrast.

“Donald Trump must be defeated, and I will do everything in my power to make that happen,” Mr. Sanders said on Wednesday. “On Sunday night, in the first one-on-one debate of this campaign, the American people will have the opportunity to see which candidate is best positioned to accomplish that goal.”