DES MOINES, Iowa — Former Secretary of State John Kerry — one of Joe Biden's highest-profile endorsers — was overheard Sunday on the phone at a Des Moines hotel explaining what he would have to do to enter the presidential race amid "the possibility of Bernie Sanders taking down the Democratic Party — down whole."

Sitting in the lobby restaurant of the Renaissance Savery hotel, Kerry was overheard by an NBC News analyst saying "maybe I'm f---ing deluding myself here" and explaining that to run, he'd have to step down from the board of Bank of America and give up his ability to make paid speeches. Kerry said donors like venture capitalist Doug Hickey would have to "raise a couple of million," adding that such donors "now have the reality of Bernie."

Asked about the call later Sunday, Kerry said he was "absolutely not" contemplating joining the Democratic primary race. He reiterated the sentiment in a tweet later, saying "any report otherwise is f---ing (or categorically) false." Minutes later, he deleted the tweet and reposted it without the expletive.

As I told the reporter, I am absolutely not running for President. Any report otherwise is categorically false. I’ve been proud to campaign with my good friend Joe Biden, who is going to win the nomination, beat Trump, and make an outstanding president. — John Kerry (@JohnKerry) February 2, 2020

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

He told NBC News later Sunday: "This is a complete and total misinterpretation based on overhearing only one side of a phone conversation. A friend who watches too much cable called me wondering whether I'd ever jump into the race late in the game if Democrats were choosing an unelectable nominee. I listed all the reasons I could not possibly do that and would not — and will not under any circumstances — do that."

It's not clear how serious Kerry was on the call about jumping into the race. But that he would even discuss the possibility suggests that prominent members of the Democratic Party remain deeply unsettled by the current field, Sanders' strength in the polls and the ability of any candidate to defeat President Donald Trump.

It also suggests that Kerry, who has campaigned with Biden in Iowa and New Hampshire, may be nervous about the former vice president's chances ahead of Monday's first-in-the-nation primary caucuses. At a campaign event in North Liberty on Saturday, Kerry spoke after Biden did, and longer.

The Biden team denied any concerns. "Everyone knows that John Kerry is all in for us," said Symone Sanders, a senior adviser to the campaign.

Kerry was the Democratic nominee in 2004, losing to President George W. Bush. He got 251 electoral votes, compared to Bush's 286.

Kerry has served as one of Biden's top surrogates in the 2020 race. Biden leads Sanders, an independent senator from Vermont, by less than 4 points in the RealClearPolitics average of national Democratic primary polls, but he trails Sanders in the Iowa and New Hampshire polling averages.

Download the NBC News app for breaking news and politics

NBC News asked Kerry last month whether he regretted not entering the 2020 race.

"I'd be a liar if I didn't say I don't come out here and have fun and your juices don't get going," he said. "But right now, they're entirely focused on helping Joe Biden become president, and I'm very happy doing what I'm doing."

Jonathan Allen reported from Des Moines, and Allan Smith reported from New York.