DENVER -- Colorado's lieutenant governor has opted out of running for the U.S. Senate, just as a former representative officially entered the race.

Jane Norton said Monday in a statement that she and her family believe she can best serve the state by continuing as lieutenant governor for the next three years.

"We have important work to do by continuing our progress in education and healthcare reform, transportation improvements and protecting and preserving the environment. I will continue to fully support Gov. Owens as he leads the state forward in these efforts," Norton said.

Her decision comes as Bob Schaffer formally launched his campaign for the seat vacated by incumbent GOP Sen. Ben Nighthorse Campbell.

Schaffer made his announcement Monday morning at his weekly breakfast with Larimer County Republicans in Fort Collins.

Citing health concerns, Campbell announced two weeks ago that he would not run for a third term, throwing the state's political landscape into turmoil.

On Sunday, the U.S. ambassador to the Vatican and former chief of the Republican National Committee, Jim Nicholson, withdrew his name from the race.

Schaffer said his team contacted Nicholson and urged him not to run. Nicholson said he's committed to being the president's ambassador to the Vatican, and wouldn't be able to get back to Colorado in time to mount a successful campaign.

Norton's announcement on Monday was the latest in a series of Republican refusals to run for the seat. In the last week and a half, Gov. Bill Owens, state Treasurer Mike Coffman, and Reps. Bob Beauprez, Scott McInnis, Tom Tancredo and Marilyn Musgrave all declined to put their name on the primary ballot.

Interior Secretary Gale Norton is the last major Republican who has not announced her intentions.

Schaffer was elected to the House in 1996. He served three terms before stepping down to fulfill a self-imposed term limit pledge.

On the Democratic side, another candidate is out of the race. Attorney Brad Freedberg announced over the weekend that he would not run.

That leaves attorney Larry Johnson, educator Mike Miles and state Attorney General Ken Salazar as the three candidates currently seeking the Democratic nomination.

Colorado's Senate seat was seen as safe for the GOP when heavily favored Campbell was in the running, but it's now a focal point for the national parties as they battle for control. Republicans hold a 51-48 majority with one Democrat-leaning Independent.