BILLINGS, Mont. (AP) — The Libertarian candidate in Montana's U.S. Senate race threw his support behind his Republican opponent Wednesday in a surprise move that came in response to an election mailer from an unknown group that appeared aimed at undermining Rosendale's support among conservatives.

Republican Matt Rosendale, Montana's state auditor and insurance commissioner, is in a tight race against two-term Democratic Sen. Jon Tester, with the balance of power in the closely divided Senate potentially at play.

Libertarian Rick Breckenridge has virtually no chance of winning and hasn't dropped out with voting already underway. His participation has threatened to peel away votes that might otherwise go toward the Republican and boost Tester's chances next week.

Montana Libertarian Party Chairman Francis Wendt disavowed Breckenridge's comments supporting Rosendale, which caught party leaders off guard. Wendt said it could make it harder for the party to qualify on future ballots if fewer voters back Libertarians on Nov. 6.

The mailer comes two days before President Donald Trump plans to hold a campaign rally in Bozeman supporting Rosendale — the president's fourth visit to the state and an indication of how much the White House wants to unseat Tester. Trump has blamed the Democrat for derailing the nomination of his first choice to head the Veterans Affairs department.

Breckenridge said in an interview that he doesn't know the source of the mailer, which promoted him as a "true conservative" and claimed that Rosendale supports using drones to spy on private citizens.

He said it appeared to be an attempt by so-called dark money groups to influence Montana's election and that he's decided Rosendale is the best candidate to stop such efforts. Federal election laws require campaign materials to disclose their funding source.

"The reality is I'm only going to get 3 or 4 percent of the vote, and he (Rosendale) has the character to combat this issue," Breckenridge said. "I'm standing in unity and solidarity with Matt to combat dark money in politics."

He said he disagreed with Rosendale on some issues, particularly the Republican's support for building a wall on the U.S.-Mexico border, and shared Tester's concerns about Supreme Court Justice Brett Kavanaugh's stance on privacy issues. But dark money in elections is more important, Breckenridge said.

The mailer is reminiscent of tactics used by Democratic-friendly groups in Tester's 2012 race to promote Libertarian candidate Dan Cox and siphon Republican voters from Rep. Denny Rehberg.

Tester won the race by 4 percentage points, and Cox captured more than 6 percent of the vote — enough to swing the election if those ballots had gone for Rehberg.

Rosendale said during a Thursday conference call with Breckenridge hosted by the Republican's campaign that he was honored to have the Libertarian's backing. Rosendale alleged that the flier was distributed by "Jon Tester's allies" but offered no proof.

Tester said the suggestion he was involved in the mailer was false.

"This whole thing about them making up a bunch of stuff about our campaign is totally bogus," the lawmaker said in a Wednesday interview. "It's just a result of a campaign (Rosendale's) that's grasping at straws. ... I felt good about where I was at yesterday and I feel good about where I'm at today."

It's uncertain who sent the mailer or how many people received it. Rosendale campaign spokesman Shane Scanlon said it went out to voters in the Billings and Flathead areas and possibly elsewhere.

The mailer's claim that Rosendale supports drones for government surveillance stands in stark contrast to an advertisement from Rosendale during a failed bid for the U.S. House in 2014. It showed the candidate shooting down a drone with a rifle.

Andrew Forcier, a Libertarian voter from Columbus, said it was distressing to see the party's nominee get behind a Republican candidate.

"From my standpoint, Rick just wanted media exposure, and this is his way to get it," Forcier said.

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