While a new president's party historically suffers losses in Congress during the midterm election, Biden, 79, faces extra scrutiny.
He and advisers have said as recently as Nov. 2 that he wants to run again, and that they're already making plans. White House officials expect him to run as well.
But a wide margin of Democratic losses would be viewed as a rebuke of Biden's presidency, and increase pressure on him to cede the role to someone else, some Democrats say.
"I think we're due for a generational shift," said Thomas Alan Schwartz, a presidential historian at Vanderbilt University. "I think the midterms could be decisive on that level. If the Democrats lose badly, I think you may see a fairly strong push for Biden to take himself out of 2024."
Biden choosing to step aside, however, raises its own thorny issues:
SO WHO'S THE CANDIDATE?
Vice President Kamala Harris is currently the Democrat's top alternate candidate, Democratic officials tell Reuters, with the majority of polls showing her second after Biden, and well ahead of most other oft-mentioned names. Michelle Obama, a voter favorite, has shown no intention of jumping into the race, and vice presidents who seek presidential nominations historically win them.
But Harris's approval ratings, once well over 50%, have languished at or below 40% in most polls. Her poor showing in the 2020 presidential race and lack of standout policy success as vice president have raised doubts she can defeat a Republican opponent.
Harris's office had no comment.
A trio of governors from deeply Democratic states - California's Gavin Newsom, Illinois' J.B. Pritzker and New Jersey's Phil Murphy - have already contacted potential donors and staff in the case that Biden stands down, according to two sources familiar with those efforts. None of the three would run against Biden in a primary, and they may defer to Harris, too, the sources said.
A senior Democrat said Newsom "has told people he won't run against Biden" or Harris. The Democrat said Newsom could change his mind about running against the vice president.
The campaign for Pritzker, who is seeking reelection, denied that he has talked to donors or potential staff.
Newsom's and Murphy's offices did not respond to requests for comment.
Multiple failed 2020 Democratic presidential candidates could pop up too, a scenario so unsettling for Democrats it was skewered as a horror movie by comedy television show "Saturday Night Live" last month.
Already, about 20 politicians rumored to have 2024 ambitions have raised over $591 million since January 2021 through their aligned political operations, the nonprofit Open Secrets reported in September, including Newsom, Pritzker, U.S. Senator Amy Klobuchar of Minnesota, U.S. Senator Bernie Sanders of Vermont and Transportation Secretary Pete Buttigieg.
Spending in the 2020 presidential election surpassed $5.7 billion, campaign finance research site Open Secrets found, more than double the amount spent in any of the last three presidential elections, thanks to small dollar donors.
Any Democratic primary contest would weaken the party's financial firepower in 2024, which could see spending jump again, campaign finance experts believe.
PRIMARY CALENDAR CHANGES
The Democratic National Committee (DNC) is close to making the biggest change to its presidential primary calendar in decades, which might affect who the party chooses for 2024.
Iowa and New Hampshire have long opened the presidential nominating process, but with populations at about 90% and 93% white, respectively, don't reflect the likely overall Democratic party electorate, estimated at 40% non-white by Pew Research.
South Carolina, Nevada or Michigan may host the Democratic primary instead. The changes may have little impact on an uncontested Biden reelection bid - after all, the president won the Democratic nomination with South Carolina's help.
But it could be seismic if he steps aside, forcing candidates to address the concerns of Black and Latino voters early, and potentially shifting the momentum of the race.
"It will have a dramatic impact. Having a more diverse early primary, including geography, will help ensure candidates address a broader range of issues and will ultimately produce a better candidate," said Karen Finney, a Democratic political consultant.
The DNC's Rules and Bylaws Committee is expected to meet in December on the issue, with a final decision by early January.
LESSONS FROM LBJ AND THE 1860s
Biden would not be the first U.S. president to decide against running for reelection.
Lyndon B. Johnson, challenged by fellow Democrats who opposed the Vietnam War, shocked the country by announcing in March of 1968, a presidential election year, not to run again. And multiple consecutive Republican presidents, beginning in the 1860s, served only one term.
The results were markedly different for the two parties, historians note. Johnson stepped down in a speech pleading for peace in South Vietnam and action from Congress on cutting the deficit, saying he couldn't devote time to "personal partisan causes" while Americans were dying overseas.
The chaotic Democratic convention and campaign that followed ended in Republican Richard Nixon taking office. "It was a mess, largely because it was so late in the game," said Jeremi Suri, professor in the University of Texas Department of History.
Republicans in the late 1800s, however, held the White House through a time of national division, widespread anger after the Civil War, and razor thin voting margins for several terms in a row, by putting up new candidates again and again.
"I think the Democratic party and the White House should not presume you need to run the same presidential candidate again to hold the White House," Suri said.
(Reporting by Jarrett Renshaw, Steve Holland, Nandita Bose and Heather TimmonsEditing by Alistair Bell and Rosalba O'Brien)
