CNN projected Tester as the winner, ending a ferocious battle in which an estimated $40 million was spent in a right-leaning state with fewer than 1 million people. The majority of those funds came from groups outside Montana.

Including the expected support of two independents, Democrats will control at least 54 seats in the Senate, compared with the current 53-47 majority.

Tester, a farmer and former state legislator, won the seat in a 3,000-vote upset in 2006. This year, he stressed his rural Montana roots and his independence from President Barack Obama, who lost the state by a wide margin even as he was re-elected nationally. Tester touted his advocacy for veterans and women’s health while attacking Rehberg as irresponsible and untrustworthy.

Rehberg, a veteran politician and real estate developer who has held the state’s single House seat for a decade, aimed to paint Tester as a true-blue liberal supporter of the president who was out of touch with Montana issues. Rehberg stressed his budget-cutting acumen and aimed to ride Mitt Romney’s coattails in a state with a strong fiscal conservative streak.

Dave Parker, a Montana State University political scientist, pointed to a big turnout in the college towns of Missoula and Bozeman and unexpectedly soft support for Rehberg among older, rural voters as key factors.

Dan Cox, a Libertarian candidate, may also have hurt the Republican, Parker said.

“The spoiler may have been Dan Cox,” Parker said, noting that Cox was drawing about 6 percent of the vote and was especially strong in rural areas that are normally heavily Republican.

The contest between two well-known incumbents was a toss-up throughout, polls showed. Both parties, along with the anonymous independent groups that have flourished since the Supreme Court threw out limits on corporate campaign spending, had long anticipated a tight contest that could decide the balance of power in the Senate – and they spent accordingly. DM

Photo: U.S. Senator Jon Tester (D-MT) speaks during an interview with Reuters in Billings, Montana August 16, 2012. Tester, a farmer and high school music teacher who won the election in 2006 in an upset over Republican incumbent Conrad Burns, faces U.S. Representative Denny Rehberg (R-MT) in the November election. Picture taken August 16, 2012. REUTERS/Robert Galbraith