Sen. Bernie Sanders’ political team has noticed a significant shift in the 80-year-old former presidential candidate in recent months: his campaign fires are burning hot.

Sanders, I-Vt., will travel to Pittsburgh on Thursday to speak at a rally for Summer Lee, one of a half-dozen hard-core progressives he has endorsed in Democratic congressional primaries. He also plans to meet with Starbucks employees in the city to show support for a store-by-store unionization campaign.

And, more than two years before the 2024 election, the outspoken Sanders is in private talks about a third run for the White House, with the caveat that he would only do so if President Joe Biden reverses course and does not run, according to sources.

In an interview, Faiz Shakir, who managed Sanders’ 2020 campaign, said that promoting progressive candidates and causes can be an end in itself or a springboard for the next race: “You can do both,” he said.

All of this would be unremarkable if Sanders were shadow boxing against the backdrop of Biden’s inevitable re-election bid.

Instead, he is simply the most openly ambitious of a growing field of Democratic candidates positioning themselves to run if Biden does not, according to interviews with more than a dozen Democratic insiders. The majority of them were either not authorized to speak on the record or insisted on anonymity in order to avoid upsetting one or more of the potential candidates.

According to Democratic sources, the list of potential candidates includes Vice President Kamala Harris, Transportation Secretary Pete Buttigieg, Sen. Elizabeth Warren, D-Mass., Sen. Amy Klobuchar, D-Minn., and California Gov. Gavin Newsom.

As vice president, Harris is widely regarded as the most likely to take over if Biden resigns.

Buttigieg will make a rare appearance as a Cabinet secretary next month when he headlines the state party’s annual fundraising dinner in Minnesota. Newsom recently chastised Democrats for their abortion messaging following the publication of a draft Supreme Court opinion that would overturn the 50-year ban on states outlawing the procedure. And last month, Warren took her party to task for failing to move its agenda faster before the midterms, implicitly castigating Biden and party moderates.

“There’s a working assumption among a lot of people that the president isn’t going to run for re-election,” said another veteran party operative who disagrees.

It is true that it is not universally held. Biden, who has run three times and flirted with a run since the 1980 election, insists on running for a second term.

Nonetheless, the squabbling reflects the party’s growing dissatisfaction with Biden’s inability — and, in some cases, unwillingness — to carry out his own agenda. Some Democratic leaders believe they could do a better job of fighting for the party’s priorities, and they are beginning to express this publicly.

Although no prospective candidate has stated it, this raises the possibility that Biden could face credible opposition in a primary. Lyndon B. Johnson was the last Democratic president to lose reelection, withdrawing from the race in the midst of the Vietnam War and collapsing party support.

While Biden remains more popular among Democrats than Johnson, the reshuffling beneath him can’t help but be unsettling.

According to the top adviser on a previous presidential campaign, potential candidates are causing harm to fellow Democrats, most notably Biden.

Waiting too long to draw a contrast with a president whose approval rating is in the low 40s and who would be counterproductive to turn himself into a lame duck any sooner than necessary poses a risk for ambitious Democratic politicians.

Three days later, Shakir, Sanders’ 2020 campaign manager, sent a memo to about 20 allies informing them of Sanders’ plans for the midterms and the possibility of him running in 2024 if Biden does not. Sanders signed off on the memo, which quickly became public.

If Sanders and Warren hadn’t become bitter rivals on the campaign trail in 2020, the one-two punch might have appeared to be a coordinated bolstering of the party’s progressive wing. Instead, Democratic insiders see them as competing for power.