He argued that having Montana represented by two Democrats -- Tester and Sen. Max Baucus -- is limiting the state, and that Montana's residents want a new approach.
"We essentially have one senator, two people voting the same way, doing the same thing," Rehberg said. "It's contrary to what Montana people want. Stop the spending, slow down the spending, find waste, fraud and abuse and right the ship."
National Republican Congressional Committee Chairman Pete Session (R-Texas) responded to the news by touting Rehberg's "conservative fiscal record," and airing confidence that Montanans would elect another Republican to replace him in 2012.
"I am confident that they will elect a new Republican member of Congress who will continue Denny's commitment for a balanced budget and government that respects individual liberty," he said in a statement. "I wish Denny, his wife Jan and their children all the best as they begin a new journey to serve Montana residents."
Tester responded to the news by saying he was confident his accomplishments in the Senate will stand up to any challenger.
"We're going to continue to do job creation for Montana," Tester told the Gazette Friday. "I've never entered this race running against anyone. We will do as we always do. Whether it's Steve Daines or Dennis Rehberg, I really look forward to comparing my record against anyone's record."
And he was not shocked to find that he might be facing Rehberg on the campaign trail.
"This honestly doesn't come as a surprise," he said.