Updated 11:23 a.m. | The Associated Press this morning declared Montana Sen. Jon Tester (D) the winner of his re-election race against Rep. Denny Rehberg (R).

The win means Democrats are guaranteed to have a larger majority in the Senate next year, with at least 54 seats. The open-seat race in North Dakota has still not been called, but Democrat Heidi Heitkamp is currently leading by nearly 3,000 votes over Republican Rep. Rick Berg. If Heitkamp wins, Democrats would have their majority expanded by two seats.

“Jon Tester knows his state like the back of his hand, and he is exactly the type of quality leader we need in the US Senate,” Democratic Senatorial Campaign Committee Chairwoman Patty Murray (Wash.) said in a statement. “Montanans saw right through the millions in attack ads from Karl Rove and others.”

With 85 percent of precincts reporting, Tester led Rehberg by about 18,000 votes. Montanans also had a third choice in the race. Libertarian candidate Dan Cox took more than 6 percent of the vote, much of that likely from voters who otherwise would have supported Rehberg.

Update:

“Senator Tester and I share an abiding love for Montana and America, a value which transcends political party or disagreements on matters of policy,” Rehberg said in a statement. “I congratulate Jon on his victory in this hard-fought campaign.”