WASHINGTON (CNN) -- Sen. Wayne Allard, R-Colorado, said Monday he will not seek reelection in 2008, honoring a pledge he made when first elected to only serve two terms.

"In an age when promises are cast away as quickly as yesterday's newspaper, I believe a promise made is a promise kept," Allard said in a press conference at the Colorado State Capitol in Denver. "The people of Colorado placed their trust in me based on my promise to them to serve no more than two terms, and it was very important to me to not dishonor that trust."

Once considered safe for the GOP, Allard's decision leaves a wide open Senate seat in the swing state that elected a Democratic senator in 2004 and a Democratic governor last year. Half of Colorado's six House seats are also under Democratic control.

Several candidates from both parties have expressed interest in running including Democratic Rep. Mark Udall and former Republican Gov. Bill Owens who stepped down in January due to term limits.