Can a prominent Democrat really challenge Biden? It’s unlikely at this point.

Forty-four years ago tomorrow, the last serious opponent of a sitting Democratic president announced his campaign before 5,000 supporters in Boston.

But that challenge from Senator Edward M. Kennedy, who ran against President Jimmy Carter for the 1980 nomination, offers little precedent for the legion of Democrats concerned about President Biden’s standing ahead of a likely 2024 rematch with former President Donald J. Trump.

In panicked text conversations and late-night bar sessions, Democrats in the political world have floated the names of ambitious rising stars in the party as possible primary challengers: Gretchen Whitmer. Gavin Newsom. J.B. Pritzker. Rafael Warnock.

But given the amount of time, planning and money a presidential campaign takes, it’s highly unlikely that any of them would run against Biden at this point. Challenging a sitting president is widely seen in politics as a career killer, and virtually all Democrats talked about as possible Biden alternatives have thrown their support behind him.

Modern Democratic politics has also emphasized the traditional early states of Iowa and New Hampshire, where Senator Kennedy traveled after announcing his campaign, in favor of a diverse group of states where Biden was strong in the 2020 primary season. Biden, and before him Hillary Clinton and Barack Obama, won the nomination largely because of their strength among black voters in the Democratic primaries.

There is also the issue of qualification for primary ballots. Deadlines have already passed in Nevada and New Hampshire. Others approach Friday in Alabama, Michigan and South Carolina. Deadlines in delegate-rich California and Florida come toward the end of the month.

A theoretical primary challenger would also need to raise tens of millions of dollars to compete with the $90.5 million that Biden’s campaign committees and the allied Democratic National Committee reported in late September. The party’s top donors are effectively aligned with Mr. Biden; a primary challenger would either have to peel off a significant portion of them in a short period of time or be wealthy enough to finance a large portion of any campaign.

Mr. Pritzker, the governor of Illinois, is helping plan and finance next year’s Democratic National Convention. Mr. Newsom, the governor of California, has offered himself to debate second-tier Republican candidates on behalf of Mr. Biden. Figures like Ms. Whitmer, the governor of Michigan, and Raphael Warnock, the senator from Georgia, have given little indication that they are at odds with the White House.

And yet Mr. Biden turns 81 this month. If there is anything enduring about his electoral numbers, it is how weak his position is in the party’s key constituencies. But as the old saying goes in politics, you can’t beat something with nothing.