Montana is known for its big skies, jagged mountains and open roads. Not so much for people.

Yet as much as $40 million is being spent to tell the state's 675,000 registered voters who to pick as senator: incumbent Democrat Jon Tester or Republican Rep. Denny Rehberg. That's almost $60 for every potential voter.

Much of the TV ad spending is being done by outside groups that do not have to identify their donors, unseen hands that may tip a race that could determine which party controls the Senate.

More than 20 dark-money groups have chimed in so far, from big shots like Karl Rove's Crossroads GPS to obscurities like Citizens for Strength and Security Fund. More TV ads — an estimated 89,000 between June 1 and Oct. 21 — have run in the Tester vs. Rehberg face-off than in any other Senate race in the country, the Wesleyan Media Project says.

"I made a joke recently to some people that I was going to sit down and watch a discussion of the Senate race, interrupted by some football," said Bowen Greenwood, the head of Montana's Republican Party. "Nobody really understands how to adapt to this new landscape yet. We're all doing this by the seat of our pants."

Greenwood's lament gets at what life has become in the new post-Citizens United, Wild West of campaign finance. The ad saturation has been highest in close Senate races, not only in Montana, but in Wisconsin, Indiana and Virginia, according to Wesleyan's study of estimates by Kantar Media's Campaign Media Analysis Group, which tracks broadcast and national cable spots. And still, despite the millions spent, Tester and Rehberg are locked in a tight race, with a recent poll showing Rehberg slightly ahead.

David Parker, an associate professor at Montana State University who is writing a book on the Tester-Rehberg Senate race, estimates that as much as $15 million more will be spent to win the seat than was spent in 2006.

Outside groups could account for half the spending on TV commercials, most of which are attack ads.

"They're just harsh and negative and mean," Parker said.

Much of the spending has been on so-called "issue" ads that ran outside the reporting window for the Federal Election Commission, but Parker has driven around the state twice to collect information from TV stations on spending.

The influx of outsiders may be turning off some voters. Liberal groups with Montana-sounding names — think "rural" and "hunters" — have been accused of being fronts for national environmental interests. A cable system pulled one negative Crossroads ad for falsehoods — rare this election season. The U.S. Chamber of Commerce drew ridicule for an ad opposing "John" Tester.

Smaller groups show not only how dark the money can be, but how odd.

A nonprofit called America Is Not Stupid, incorporated last year in Florida, is among those running ads in Montana. The IRS has no record of the group's tax-exempt status. Its purported president, Miguel Angel Gutierrez, was not findable, and the lawyer who founded it, Gene Peek, didn't return phone calls. Its ad features a baby in need of a diaper change.

"I don't know what smells worse: my diaper or Jon Tester," the voiceover says.

Greenwood said a constituent had called to ask if America Is Not Stupid was affiliated with the Republican Party. The GOP state director said he'd never heard of the group.

"Politicians care a great deal about their message," he said. "And nobody is happy that somebody else is dictating what we're talking about rather than our candidates."