In Eastern Nebraska, far from the presidential battleground states, a drama is playing out that could, in a perfectly plausible November scenario, have history-altering repercussions for the nation’s future and the next president — and it may all come down to one man.

A single Republican state senator from Omaha, Mike McDonnell, has so far stood firm against a push by former President Donald J. Trump, national Republicans and the Nebraska G.O.P. to change Nebraska from a state that divides its electoral votes by congressional district to one that awards all of them to the statewide winner. Maine is the only other state without a winner-take-all system.

If Mr. McDonnell buckles, two other Republican senators in Nebraska’s unicameral legislature who have also not yet committed to changing Nebraska’s system are likely to follow his lead, according to a number of Republicans and Democrats involved in the discussions going on at the State Capitol.

The tumbling dominoes would almost certainly give the single electoral vote of Omaha and its suburbs, which Vice President Kamala Harris is favored to win, to Mr. Trump.

That might not sound like much, but if Ms. Harris were to win the so-called blue wall — Pennsylvania, Michigan and Wisconsin — while losing every other battleground state, Nevada, Arizona, Georgia and North Carolina, that one electoral vote would be the difference between a 270-268 Electoral College victory for the vice president or a 269-269 tie. And in the event of a tie, the House of Representatives would determine the winner, not by raw votes of House members but by the support of each state delegation.

With more delegations in Republican control, Mr. Trump would almost certainly win.

As of Friday, Mr. McDonnell, who is barred by Nebraska’s term limits law from seeking re-election, had not changed his position.

Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.