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...