A third-party Senate candidate in Montana has effectively dropped out of the race and endorsed Republican candidate Matt Rosendale in his bid to unseat Sen. Jon Tester (D., Mont.).

Rick Breckenridge, a Libertarian and a surveyor, cited his anger with an anonymous campaign mailer encouraging voters to choose him over Rosendale because of privacy concerns. The Republican nominee said Tester's allies were behind the message, and he said he was honored to have Breckenridge's backing.

"I am here to support Matt and his candidacy," Breckenridge said at a news conference. "And to support him in the continuing effort to be the front man in the cause of liberty."

Sending campaign mailers without a name is a violation of state and federal election laws, The Great Falls Tribune reported.

Breckenridge estimated in an interview that he would get 3-4 percent of the vote in the final tally and perhaps swing 1 percent of the votes to Rosendale, and an expert who spoke with the Falls Tribune said it was unlikely the decision would sway the results with hundreds of thousands of ballots already mailed in.

His name will still be on the ballot on Tuesday.

Tester's campaign denied having anything to do with the anonymous mailer.

"This is yet another lie from Matt Rosendale – our campaign had nothing to do with this mailer, and Jon believes all campaign expenditures should be disclosed," Tester spokesman Chris Meagher said. "We condemn this mailer and any other efforts like it."

Tester has held a consistent, small lead in polls over Rosendale, but he's running in a state President Donald Trump carried overwhelmingly in 2016. Democrats cannot afford to lose any incumbents if they hope to take back the Senate majority, but they're defending far more seats than Republicans in the 2018 map.