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By the time Tester and Rehberg started buying TV ads, outside groups had been defining the race for a year.

Rehberg, 57, a six-term congressman and rancher often pictured wearing a cowboy hat and a plaid shirt, was portrayed as voting five times to increase his pay and charging an SUV to taxpayers. Tester, 56, a farmer with a flat top, was dinged for voting with Obama 95 percent of the time.

Tester's campaign went up with ads in March, mainly to counter the outside messages.

"The original plans were going up 60 or 90 days later than that," Tester said. "But it was important...We had to remind people of who I am."

His early ads highlighted his Montana roots, depicting him riding a combine on his farm and packing up Montana beef to carry back to Washington.

Rehberg had less money, so his earliest TV ads, which mainly attacked Tester, went up in May.

Neither Rehberg nor anyone from his media staff responded to requests for an interview on his views on campaign finance. In the past, he has said he supports theCitizens United ruling.

Meanwhile, conservative groups bought TV ads that hit at Tester but stopped just short of telling people how to vote. For instance, the conservative 60 Plus Association spent almost $500,000 buying TV ads featuring crooner Pat Boone criticizing Tester over the health care law. None of that was reported to the FEC.

Over the summer, the Concerned Women for America's legislative committee, Crossroads GPS and the U.S. Chamber of Commerce all weighed in. The TV spots were overwhelmingly negative, and many of them were cookie-cutter ads, similar to those that ran in other states against Democrats.

Liberal groups bought TV ads, too, but that was only part of their game plan. They spent their dark money on retail politics, hitting the streets and knocking on doors.

In January, the League of Conservation Voters set up two offices in Montana — one in Missoula and one in Billings. It canvassed voters and hired a full-time organizer, reaching out to 28,000 sporadic voters to urge them to vote early by mail.

Lindsay Love, the spokeswoman at Planned Parenthood Advocates of Montana, another nonprofit that doesn't report its donors for election spending, said the group targeted 41,000 female voters. More than 1,500 people ended up knocking on 28,500 doors and making 162,000 phone calls, she said. The group sent out about 470,000 pieces of mail.

"It's hard to unpack this," Parker said. "But it's fascinating to look at groups like the League, unions and Planned Parenthood. By and large, they did phones, canvassing, mail, very little TV. One of the best ways to get out the vote is personalized contact."

Many liberal groups active in Montana, including Montana Hunters and Anglers, were connected through Hilltop Public Solutions, a Beltway consulting firm.

Barrett Kaiser, a former aide to Montana's other Democratic senator, Max Baucus, is a partner at Hilltop and runs its office in Billings. The Hilltop website notes that Kaiser helped with Tester's upset Senate win in 2006. Kaiser is also a good friend of Messina, the manager of Obama's 2012 campaign, who also once worked for Baucus.

Kaiser was on the board of the Montana Hunters and Anglers dark money group. Another Hilltop employee in Billings served as the treasurer for the Montana Hunters and Anglers super PAC.

Hilltop partners in Washington also helped run two other dark money groups that spent money on the Montana race: the Citizens for Strength and Security Fund and thePartnership to Protect Medicare.

The League of Conservation Voters and Planned Parenthood Advocates of Montana paid management fees to Hilltop.

No one from Hilltop returned calls, but Nayak and Love said they worked with Hilltop independently of other groups.

Outside groups are allowed to coordinate with each other or use the same consultants — they're just not allowed to coordinate with a candidate. By working together, groups can disguise who is actually behind an ad.

In early July, for instance, the League of Conservation Voters gave $410,000 to the Montana Hunters and Anglers super PAC — almost all the money the group raised as of that date.

When the super PAC spent the money on TV ads against Rehberg later that month, the spots were paid for by what appeared to be an organization of Montana hunters, not some Washington-based conservationist group. Nayak said that was not a coincidence.

"We figured having a local brand like that and partnering with them on local issues made more sense than having a D.C. brand," he said.

Nayak said the League did not donate money for the later ads pushing Cox, the libertarian.

It's not clear where that money came from. The dark money side of Montana Hunters and Anglers paid for the radio ads. The super PAC bought the TV ads and had to disclose its donors, but FEC filings show its money came mainly from two other super PACs, which in turn reported getting most of their money from unions and dark money groups, including the League.