BISMARCK, N.D. (AP) — North Dakota Rep. Kevin Cramer faces a little-known opponent in the primary election as he looks forward to what's expected to be one of the nation's top Senate races in the fall.

Cramer and Democratic Sen. Heidi Heitkamp have been stockpiling millions for a race that Republicans see as one of their top chances to gain ground in the chamber.

Cramer's opponent Tuesday is Thomas O'Neill, an Air Force veteran who didn't mount a major campaign.

The other notable statewide race is for Cramer's U.S. House seat. State Sen. Kelly Armstrong was seen as a heavy favorite over a pair of lesser-known and lightly funded opponents.