A single Republican state senator appears to be holding back a push by Donald J. Trump to net a potentially pivotal electoral vote even before ballots are cast.

Nebraska is one of two states that award some of its electoral votes by congressional district. A vote from the Omaha area is part of Harris’s easiest path to victory.

Too late for Maine Democrats to retaliate, Nebraska Republicans may implement a winner-take-all system, swiping an electoral vote from Harris.