The Denver Post reported on Wednesday that Gardner would run for Senate. Gardner's decision to jump into the race means that, if Gardner wins the nomination, Udall will face a strong Republican challenger in his re-election campaign.
Gardner, considered a rising star within the GOP, plans to formally announce his candidacy in the next few days according to the Post.
The Colorado field joins a field of candidates running for the GOP nomination. Other candidates in the Senate primary includes Weld County District Attorney Ken Buck, who won the GOP nomination for Senate in 2010, and state Sen. Owen Hill and state Rep. Amy Stephens.