one-horse race

noun2

1. •
   An election campaign or other competitive situation in which only one competitor is entered or in which only one competitor has a realistic chance of winning.

   • Although Mr Balladur is far ahead in the opinion polls, the spring election is far from being a one-horse race. — 1995
2. •
   (informal) A horse race in which a single horse takes such a considerable lead that the other horses are no longer contenders to win.

   • Sinister Minister . . . kept extending his lead and turned the stretch run into a one-horse race. — 2006