Somebody is trying to upset Montana's Senate race. The problem is, nobody can figure out exactly who it is.

Montana is seen as a key to Republicans winning back control of the Senate -- barring upsets. Four-term Republican Rep. Denny Rehberg is challenging Jon Tester, incumbent Democratic junior senator, who barely won in 2006. Rehberg already represents the entire state as its at-large member of Congress, so name recognition is no problem.

Polling is extremely tight. Rehberg's slight but consistent lead serves as a reminder that every vote counts. Rasmussen Reports shows Rehberg 4 percentage points ahead (47 to 43); Democrat-leaning Public Policy Polling shows Tester 2 points ahead (45 to 43); and RealClearPolitics, which averages several polls, gives Rehberg a scant 1.7-point spread (46 to 44.3).

But two weeks ago, Twitter came alive in Montana with word of a spoiler. Mike Miller, a Republican state representative, tweeted, "Wow! Just got a survey call advocating for [Libertarian Party Senate candidate] Dan Cox. Asked who I was voting for then read a couple statements from Cox."

A constituent tweeted back to Miller, "Mine was earlier today too. Never even heard of him, so it was a surprise to me." Most others said it sounded like the caller was only trying to convince them not to vote for Rehberg.

An investigative journalist from Helena asked, "Any idea who funded the calls?" Miller replied, "No idea on funding." The Libertarians don't have the money. Third-party friends of Tester became the prime suspects.

A Public Policy Polling release reinforced the suspicion: "Tester really needs Cox to hold onto his voters because if he doesn't, they're a whole lot more likely to end up in the Rehberg camp than with Tester."

A week ago, headlines blaring, "Libertarian Could be Senate Race Spoiler" spread the Cox story across the state. "Republican-leaning voters say they are receiving unsolicited calls that tout the conservative credentials of the Libertarian candidate ... at the possible expense of the GOP contender."

The Rehberg campaign smelled a last-ditch effort to deceive voters. Rehberg spokesman Chris Bond told me, "Sen. Tester's own liberal allies appear to be acknowledging what Tester himself refuses to admit, that he cannot win re-election based on his record of 95 percent support for President Obama's job-killing policies." But Bond had no idea who was behind the calls.

The calls had come from a "nuisance number" available for anonymous fundraising pitches and "push polls" -- pushing voters from one candidate to another. The number's log showed a similar campaign from May 2012, evidently run by the League of Conservation Voters. Montana Conservation Voters has endorsed Tester and named Rehberg to their "Dirty Dozen," enough motive to sabotage Rehberg's campaign.

I called Theresa Keaveny, executive director of Montana Conservation Voters, and asked if her group was behind the calls in any way. Keaveny said she had not heard of the calls and gave an unequivocal denial of any MCV involvement.

The calls were possibly just an ordinary candidate canvassing for voters, so I did a background search on Cox -- he lives in Hamilton, runs an online fishing tackle store and has not filed required Federal Elections Commission reports -- which he needn't do unless he raises $5,000 or more.

I asked Cox if he was behind the calls. He told me, "No, and I am not trying to split the Republican vote. Besides, somebody, probably Rehberg allies, is calling my supporters trying to talk them into voting for Rehberg."

Why hadn't he filed with the FEC? "We haven't reached the $5,000 threshold, so I'm exempt."

Will the tangled calling campaign influence the outcome of the election? Bond doesn't think so. He said, "Montanans are smart, and they're not going to fall for this sort of deceptive politics coming from Sen. Tester's side."

Tester's campaign publicly denies any part in it. The mystery deepens.

Examiner Columnist Ron Arnold is executive vice president of the Center for the Defense of Free Enterprise.