No matter which party wins on Tuesday, the Senate majority is likely to be tight.
The midterm elections could yield a narrow majority for either party or even a 50-50 tie, which could prompt a party switch that would reshuffle the balance of power. There’s a long-shot chance of a scenario that hasn’t happened in decades, in which an independent senator could refuse to align with either party and keep control up for grabs. And there is a possibility that one or more election challenges could keep seats unfilled for weeks or months, leaving the majority uncertain.
Any one of those results has the potential to change how the Senate functions and its ability to pass legislation and confirm President Trump’s nominees. And those effects would intensify if Democrats were to take control of the House, as polling has indicated they are positioned to do. That would give them the ability to send legislation to the Senate to force a confrontation with Republicans or pick veto fights with the president.
Here is a look at various possibilities and their consequences:
A slim majority would empower the middle.
Based on polls and extensive reporting, Democrats appear to be in position to gain seats in the Senate, where they now hold 47 seats and Republicans hold 53. Few strategists in either party expect the G.O.P. to expand its majority, but Democrats could increase their numbers by just one or two, leaving Republicans in charge.
The tightening of the margin would make life more difficult for Republican leaders and give added leverage to those willing to break with their party and Mr. Trump. They include Senators Lisa Murkowski of Alaska, who has been attacked repeatedly by Mr. Trump, and Susan Collins of Maine, if she survives her own re-election battle.
An even or closely divided Senate has historically put pressure on lawmakers who are considered potential party switchers to make the move and swing the majority.
Should Republicans end up with 49 seats, they would almost certainly try to persuade Senator John Fetterman of Pennsylvania, a Democrat who has expressed frustration with the party and the Senate itself, to flip and hand them back the majority. Mr. Fetterman has said repeatedly, however, that he does not intend to switch.
A 50-50 tie would tie up Vance.
If each party winds up with 50 seats, Republicans would retain control by virtue of Vice President JD Vance’s power to break Senate ties as the presiding officer. But as with recent evenly divided Senates in 2001 and 2021, the two parties would be expected to work out a power-sharing agreement likely to include equal committee representation and equity in staff and space.
The Republican leader would still control the legislative agenda, but the opposition of a single majority senator could upend plans while both parties would have zero room for defections or member absences.
Mr. Vance, expected to be a leading Republican candidate for president in 2028, would find himself tied to the Senate because of the need to be available to break ties.
Should this outcome materialize, Democrats would likely lobby Ms. Murkowski, the Alaska Republican attacked by Mr. Trump, to cross the aisle or become an independent. It would be a tough sell, as she has powerful committee positions as a Republican.
But such switches have happened before. Jim Jeffords, then a Republican senator from Vermont, became an independent in 2001 and began caucusing with Democrats, handing them the majority.
An independent could scramble control.
At least three independent Senate candidates have declared they would not align with either party should they be elected. None is favored to win, but control of the chamber could remain up in the air if one or more of them prevailed and the chamber was otherwise split between the two parties.
The Senate’s two current independents caucus with Democrats. But three running as independents this year — Todd Achilles in Idaho, Seth Bodnar in Montana and Dan Osborn in Nebraska — say they would not support the chosen leader of either party. They say that stance would force both Republican and Democratic leaders to negotiate with them on legislation and committee assignments and give them significant leverage in a closely divided Senate.
The Senate has not faced such a situation since 1952, when Senator Wayne Morse, Republican of Oregon, declared himself an independent, later moving a folding chair into the center aisle of the Senate. He was denied committee seats and ultimately joined the Democratic Party.
Independents with no party alignment would represent uncharted territory for the modern Senate.
Election challenges could leave seats unfilled.
With so much at stake, challenges to seating apparent election winners could occur, though such disputes have been rare in the Senate in modern times.
Republicans already appear to be preparing to contest the potential election of Mary Peltola, the Democratic candidate in Alaska, based on reports by The New York Times that her campaign recruited a candidate whose name is similar to that of Senator Dan Sullivan, the Republican incumbent, to try to dilute his support and boost her candidacy. A protracted fight could leave the seat open and the balance of power in the Senate hanging.
Democrats say they have been preparing for months for such election disputes given Mr. Trump’s pledges to question election results. The president has already said Ms. Peltola, a former House member, should be removed from the ballot.
Senator Al Franken, Democrat of Minnesota, was not sworn in until more than eight months after his 2008 election, after a long recount and court battle.
The last Senate fight over seating a member was in 2009, when Democrats threatened to bar Roland Burris, Democrat of Illinois. Mr. Burris had been appointed by Rod Blagojevich, then the Illinois governor, who was under federal investigation for his handling of the Senate vacancy left by Barack Obama’s election to the presidency.
Mr. Burris was eventually allowed to serve.
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