Outside groups are raining media dollars on Montana, which is home to one of this cycle’s marquee Senate races.

Sen. Jon Tester (D) and his Republican opponent, Rep. Denny Rehberg, have been inundated with hundreds of thousands of dollars’ worth of TV ads, including several just in the past month. The spending is emblematic of the state’s relatively inexpensive media markets and the race’s competitiveness and potential to swing the Senate majority. It also puts the Montana race on track to be among this cycle’s most expensive in terms of spending by outside interest groups.

Sources in the state disagree on exactly how much has been spent against each candidate. According to a Republican-compiled breakdown of spending that was obtained by Roll Call, overall outside spending in Montana has already eclipsed $2 million.

That includes $911,000 in anti-Rehberg or pro-Tester ads and $614,000 in anti-Tester or pro-Rehberg ads. An additional $524,000 in spending is listed as “issue ads,” including $80,000 in anti-President Barack Obama ads from Crossroads GPS.

Democrats in the state have a different count: $700,000 in anti-Tester ads and $445,000 in anti-Rehberg ads. Either way, the heat is on.

Montana is one of several battleground states receiving early attention from outside groups. The U.S. Chamber of Commerce recently launched a six-state ad campaign in Iowa, Montana, Nevada, Ohio, Pennsylvania and Washington.

“This is the earliest we’ve gone up in our voter education efforts,” chamber spokesman J.P. Fielder said. “We think that the voters need to know how the decisions being made in Washington [D.C.] are impacting their pocketbooks.”

The chamber’s Montana ad urges viewers to tell Tester to stop supporting “big government” policies that result in Medicare cuts, rising health care costs and more unemployment.

Crossroads GPS also launched a $157,000 TV ad buy this month across the state’s five media markets that accused Tester of supporting a bill allowing the Environmental Protection Agency to regulate farm dust, something the Tester campaign vehemently denies.

“As a 30-year politician, [Rehberg] encourages politics-as-usual ads that go as far as attacking Jon for a vote that never even happened,” Tester campaign manager Preston Elliott said in a statement to Roll Call. “Montanans deserve better, which is why Jon has a strong record of demanding laws to make our elections transparent.”

The American Federation of State, County and Municipal Employees; the Service Employees International Union; and Americans United for Change recently joined forces to launch an ad against Rehberg that targets senior voters.

“I’ll remember you cut Medicare if I fall or get hurt,” an older woman’s voice says in the ad. “I’ll remember you protected millionaires over protecting my health. My friends will remember it too.”

There are state-based groups getting involved as well. Montana Hunters & Anglers Action, a 501(c)(4), spent more than $200,000 on two TV ads criticizing Rehberg for supporting a bill “that gives the Department of Homeland Security complete control over millions of acres of Montana public lands,” as the announcer said in one ad.

The Rehberg campaign argues that voters will focus on Tester’s support for Obama’s policies and Supreme Court nominees, not outside groups attacking the six-term Congressman.

“Montanans aren’t going to be distracted by false attacks from left-wing activist groups,” Rehberg adviser Erik Iverson said. “They know Denny offers real solutions while Jon Tester votes for President Obama’s agenda 97 percent of the time. At the end of the day, that’s the fundamental difference between the candidates, and it will be the difference in this election.”

The Montana Hunters & Anglers group is led by Land Tawney, a senior manager at the National Wildlife Federation; George Cooper, a communications adviser and former CNN producer; Kendall Van Dyk, a Democratic state Senator; and Barrett Kaiser, a former longtime aide to Sen. Max Baucus (D-Mont.).

In an interview, Tawney would not disclose the group’s donors, but he said that as long as the group can raise money, it plans to continue producing ads next year.

“It’s a way of life and something we hold dearly, and I think we want to make a difference on issues,” Tawney said of the group’s focus on outdoor sporting. “Depending on what those issues are and what kind of traction we can get on fundraising, I think we’ll be planning around issue-based stuff.”