Former Republican senator Larry Pressler will run for the Senate in South Dakota again, this time as an independent, he announced Thursday, officially inserting a new dose of uncertainty into a race that represents the GOP's best pickup opportunity.

"Today, I am announcing that I am running for the United States Senate, and I intend to win," Pressler said, according to the Sioux Falls Argus Leader. Pressler had hinted earlier this year that he was probably going to run.

Pressler served in the Senate from 1979 to 1997. He is a moderate who supported President Obama in 2008 and 2012.

The former senator joins a field that includes front-running former governor Mike Rounds (R) and Democrat Rick Weiland, among others. Pressler's history of winning over Republican voters could be a cause for concern for Rounds, who right now stands as the clear favorite in the race for retiring Democratic Sen. Tim Johnson's seat.

"I'm going to say what I believe and I hope to have support across the board," Pressler said, according to the Associated Press.