DENVER -- Former Rep. Bob Schaffer on Friday became the first Republican candidate in the race to succeed two-term GOP Sen. Ben Nighthorse Campbell, who is stepping down this fall because of health concerns.

Schaffer announced his plans just hours after Reps. Scott McInnis and Tom Tancredo joined the growing list of Republicans who have declined to run. Schaffer said he will make a formal announcement next week.

"I've filed and I have an organization that I've put together, an exploratory team and a team that is getting larger," he said. "I see nothing that discourages me."

Schaffer served three terms in the House beginning in 1998, stepping down to fulfill a self-imposed term limit pledge.

Before his announcement, the GOP was a party in search of a candidate.

McInnis said he would not run for Senate but will instead join the Denver law firm of Hogan and Hartson after his fifth term in the House ends in January. Coincidentally, the managing partner of that law firm is Tom Strickland, a former Democratic candidate for U.S. Senate. Strickland lost a close race to Wayne Allard in November 2002.

"My decision not to run for the Senate comes after much soul-searching with my family, friends and supporters," McInnis said. "I can assure you this was among the most difficult choices I have ever had to make."

McInnis' announcement was preceded by Tancredo's declaration that instead of running for Senate, he will run for re-election to his House district in the southern and western suburbs of Denver. He said he can best achieve his goals by staying in the House.

"The people of the 6th District elected me to serve as their congressman, and that's exactly what I plan to do," Tancredo said.

And despite pressure from Republican party leaders, Gov. Bill Owens, Rep. Bob Beauprez and state Treasurer Mike Coffman all announced this week that they would not run for the open seat.

Owens said Friday that several other Republicans are considering the race and the current GOP list is not complete.

"I think there is going to be another announcement soon," Owens said. "We will have a very competitive candidate or candidates running."

Other potential GOP candidates include Lt. Gov. Jane Norton and Jim Nicholson, the chairman of the Republican National Committee from 1997 to 2001 and who helped raise millions for the party and President Bush's campaign.

Schaffer said his team contacted Nicholson and urged him not to run.

"The timing is not right for him," Schaffer said.

On the Democratic side, state Attorney General Ken Salazar remains the biggest name in the race so far. In fact Rep. Mark Udall and millionaire Rutt Bridges bowed out of the race on Wednesday to support him.

Salazar is one of his party's brightest hopes of retaking the majority in the U.S. Senate.

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.

Many political observers had expected Salazar to run for governor when the term-limited Owens steps down in 2006 but he said the same issues that interested him in the governor's office are behind his run for the Senate.

"I care a lot about our natural resources and about the beauty of Colorado," Salazar said.

He wrote the constitutional amendment that created Great Outdoors Colorado, which uses state Lottery funds for parks, trails, open space and recreation, and was the program's first chairman.

"Our quality of life is very much dependent on our being able to maintain our natural heritage," Salazar said.

El Paso County educator and former Army Ranger Mike Miles, Boulder lawyer Larry Johnson and Denver lawyer Brad Freedberg are the other Democrats in the race but Salazar remains the heavyweight in the contest.