Control of the Senate now hangs on Montana and Virginia, where races have not yet been called.
Democratic challenger Jon Tester started out ahead in the Montana count, but his lead over Republican incumbent Sen. Conrad Burns has since dwindled.
The race remains too close to call, and unspecified problems with tabulation machines in Yellowstone County have slowed down the count. Tabulation of votes had to be started over in the county, which is one of Montana's biggest and also considered to have a strong Republican base.
Final results are expected sometime this morning.