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The funny thing is, pre-2020 election, the Biden campaign seemed to indicate that he was planning only on a single term:

March 2019:

Mr. Biden and his top advisers are considering nodding to the rising next generation in Democratic politics — and elevating an heir — by announcing a running mate early, well before the nomination is sealed. Also under discussion is a possible pledge to serve only one term and framing Mr. Biden’s 2020 campaign as a one-time rescue mission for a beleaguered country, according to multiple party officials.

December 2019:

Former Vice President Joe Biden’s top advisers and prominent Democrats outside the Biden campaign have recently revived a long-running debate whether Biden should publicly pledge to serve only one term, with Biden himself signaling to aides that he would serve only a single term.

While the option of making a public pledge remains available, Biden has for now settled on an alternative strategy: quietly indicating that he will almost certainly not run for a second term while declining to make a promise that he and his advisers fear could turn him into a lame duck and sap him of his political capital.

According to four people who regularly talk to Biden, all of whom asked for anonymity to discuss internal campaign matters, it is virtually inconceivable that he will run for reelection in 2024, when he would be the first octogenarian president.

“If Biden is elected,” a prominent adviser to the campaign said, “he’s going to be 82 years old in four years and he won’t be running for reelection.”

April 2020:

And Mr. Biden himself has increasingly pushed into the political foreground the overwhelming reason that his choice may be the most consequential in decades: the expectation, downplayed but not exactly denied by the Biden campaign, that the 77-year-old would be a one-term president. If that turns out to be the case, his running mate now could well be leading the Democratic ticket in four years.

“I view myself as a transition candidate,” Mr. Biden said during an online fund-raiser last week, likening his would-be presidential appointments to an athletic team stocking its roster with promising talent: “You got to get more people on the bench that are ready to go in — ‘Put me in coach, I’m ready to play.’ Well, there’s a lot of people that are ready to play, women and men.”