Former Alaska Senate nominee Joe Miller (R) promised he'll back the winner of next Tuesday's Republican primary, ending Republicans' fears that he'd run as an independent or libertarian candidate and hand the race to Sen. Mark BegichMark Peter BegichPerez creates advisory team for DNC transition The future of the Arctic 2016’s battle for the Senate: A shifting map MORE (D-Alaska).

"If it's one of you two guys — I've never said this before, I'll support you guys," Miller said in a Thursday night debate. "I will. We've got to get rid of Begich, there's no question about it."

Miller had refused for months to say that he'd back either former Alaska Department of Natural Resources Director Dan Sullivan or Alaska Lt. Gov. Mead Treadwell if either of them won the Republican primary. The reversal could prove a major shift in the race, and potentially for control of the Senate.

Republicans had grown increasingly worried that Miller would either run a write-in campaign or convince his ally who'd won the Libertarian Party nomination to step aside for him, pulling votes away from their nominee and giving Begich a huge boost in what is otherwise expected to be a tight contest.

Miller had repeatedly and pointedly said he wouldn't endorse either Sullivan or Treadwell because they'd both backed Sen. Lisa MurkowskiLisa Ann MurkowskiProposed budget for Indian Health Services won't treat Native American patients equally Keep anti-environment riders for Alaska out of spending bill Industry should comply with the Methane Waste Prevention Rule MORE (R-Alaska) when she ran as a write-in candidate in the general election after he defeated her in the 2010 primary.

"We're not making commitments with those who divided the Republican Party in 2010," he told The Hill in late June.

But his newfound position could give the eventual nominee, which most Republicans expect will be Sullivan, a better shot to unseat Begich.