Colorado's race for the U.S. Senate was thrown into disarray when Republican Ben Nighthorse Campbell (search) announced he would not seek a third term this fall.

Campbell, 70, made the surprise announcement Wednesday, citing declining health. He was treated for prostate cancer last year.

His Washington office also faces allegations that a longtime aide had taken kickbacks.

Campbell's decision gave Democrats another open Senate seat to target in November and threw the Colorado Senate race wide open. Pollsters suggested heavyweights like GOP Gov. Bill Owens (search) and former Democratic Sen. Gary Hart (search) might get into the race, but there was no immediate word from them. Hart earlier declined to seek the seat.

Democratic Rep. Diana DeGette (search) said she was weighing the possibility of running at the request of party leaders.

Campbell's decision will make it harder for Republicans to build on their 51-48 Senate majority and means they will have to spend more money on the Colorado race than planned.

"Obviously, Senator Campbell's announcement is a major setback for Republicans," said University of Virginia political scientist Larry Sabato.

Speculation about Republican candidates immediately turned to Owens, a popular politician who rolled to a convincing re-election victory in 2002 but can't seek a third term in 2006 because of term-limits. Owens issued a statement thanking Campbell for his years of service but did not comment on his political plans.

Another potential GOP contender is Rep. Scott McInnis (search), a six-term congressman who announced last year he would not seek re-election. McInnis, who still has nearly $1.5 million in his campaign account, said Wednesday he must talk to his wife before making a decision.

Rep. Tom Tancredo (search) and State Treasurer Mike Coffman (search), both Republicans, also have expressed interest in the seat.

Five relatively unknown Democrats have announced they'll run, and Campbell's retirement raised the possibility that other, higher-profile Democrats would enter the race.

Democratic U.S. Rep. Mark Udall (search) declined to say if he was reconsidering his decision not to run.

Campbell's turnabout came four months after he announced his re-election campaign at a Denver fund-raiser attended by Vice President Dick Cheney.

At the Nov. 6 campaign kickoff, Campbell said his prostate cancer was under control. But on Wednesday, he cited his cancer treatments and his two recent nights in the hospital, one for chest pains that turned out to be heartburn.

"After spending another night in the hospital, I realize that deteriorating health may hamper my ability to serve," he said.

Campbell was first elected to the Senate in 1992 as a Democrat and switched parties three years later. A Northern Cheyenne tribal chief, he cuts a distinctive figure in the Capitol, sporting a ponytail and ornate jewelry and riding a Harley-Davidson to work.

Campbell's office was rocked last month when a former staffer alleged that another Campbell aide, Ginnie Kontnik, had taken kickbacks. The senator said he knew nothing about any wrongdoing and reported the matter to the Senate ethics committee.

Kontnik has denied getting kickbacks and described the payments as reimbursements for legitimate expenses.

President Bush in a statement hailed Campbell as "a leader in seeking the protection of our public lands and natural resources, and a tireless champion for Native Americans."