Colorado Republican Sen. Wayne Allard announced Monday that he will not run for re-election in 2008.

Allard said he would honor a pledge he made in 1996 to serve only two terms.

“When I first announced for Congress, I made a pledge to be a citizen legislator and serve in the Jeffersonian ideal of public service where you serve for a period of time and then return to live under the laws you have helped enact,” said Allard. “I also pledged to the people of Colorado, that if elected, I would serve no more than two terms in the United States Senate. Today, I am honoring both of those pledges to the citizens of Colorado.”

The decision sets up a wide-open race. Allard's seat was once considered safe for the GOP, but Colorado voters have shown a penchant lately for replacing Republicans with Democrats.

Democrats see the race as a chance to pick up another vote in Congress after wresting two House seats and a Senate seat from Republicans the past two years.

Democratic Rep. Mark Udall, a popular five-term congressman and son of former U.S. Rep. Morris Udall of Arizona, has said he will run for the seat whether Allard does or not. Another Democrat who has been mentioned as a potential candidate is Denver Mayor John Hickenlooper.

On the Republican side, former congressman Scott McInnis said he will run if Allard and former Gov. Bill Owens don't. Owens, a Republican, left office last week after term limits prevented him from running again.

Another prominent Republican, anti-immigration firebrand Rep. Tom Tancredo, also has been mentioned as a possible candidate. So have Attorney General John Suthers, who said through a spokeswoman it was premature to make a decision about 2008, and John Elway, the former Denver Broncos quarterback.

Allard's political strength came into question in 2006 when Time magazine ranked him as one of the five worst U.S. senators.

"In a Senate full of ambitious members, Colorado Republican Wayne Allard is so bland that his critics have dubbed him 'Dullard,"' Time said.

The article criticized him for "almost never" playing a major role in legislation despite being a Republican on the powerful Budget and Appropriations committees.

Allard's chief of staff, Sean Conway, criticized the Time ranking as "more like a popularity poll" and said it was based mostly on opinion.