At a time when he should be cruising to a third term, Sen. Ben Nighthorse Campbell (R-Colo.) is instead grappling with the potential political fallout from accusations that his just-departed chief of staff demanded kickbacks from an underling.

Campbell must now prepare for battle without the services of his most loyal lieutenant as Democrats, previously dispirited by their inability to find a top-tier challenger, appear energized by the latest revelations about him — and by the prospect of Rutt Bridges, a wealthy philanthropist and venture capitalist, entering the Senate race.

“Irrespective of anything that’s going on right now — and allegations of public corruption are grave — we’ve made the case from day one that Colorado is a state that we feel we can win and that Senator Campbell is vulnerable,” said Brad Woodhouse, a spokesman for the Democratic Senatorial Campaign Committee.

Even before the latest revelations, national Democrats have kept up a merciless drumbeat of attacks on the party-switcher’s record and behavior. There have been ongoing whispers that the 70-year-old Senator, despite protestations to the contrary, might rather retire. Then there was Campbell’s reluctant admission last fall that he had prostate cancer — a revelation that made the retirement rumors grow louder.

Campbell’s announcement Friday that his longtime chief of staff, Ginnie Kontnik, was resigning, appeared to climax several weeks of intrigue and speculation in both Washington and Denver about her status.

Democrats immediately pounced on the news, suggesting that a staff shakeup could damage Campbell’s re-election prospects.

But rather than the resignation resolving anything, the matter became even more explosive when The Denver Post reported Sunday that another former Campbell staffer, Brian Thompson, had accused Kontnik of demanding a $2,000 kickback from him in 2002.

Thompson said Kontnik had arranged for bonuses for him but demanded a portion of it so she could pay her divorce lawyer. In a statement to the newspaper, Kontnik, who had worked for the Senator since 1992 and has been his top aide since he switched parties in 1995, admitted she took the money but said it was for legitimate reimbursements. She added that she had made Campbell aware of the arrangement.

In the wake of that report, Campbell’s office said it would forward the allegations to the Senate Ethics Committee — which may not have jurisdiction now that Kontnik has resigned. But Democrats and “good government” groups are clamoring for a federal investigation.

“This is not just about Senator Campbell’s chief of staff demanding kickbacks from her employees,” said Chris Gates, chairman of the Colorado Democratic Party. “Ginnie Kontnik has made it clear this is also about Senator Campbell’s willingness to approve an obviously illegal arrangement.”

Aides to the Ethics Committee declined to comment Monday on the Campbell case, declining to comment on whether Campbell had officially sent any letters or paperwork formally requesting an investigation into the alleged payback scheme.

Because she has left the Senate payroll, Kontnik no longer faces any potential retribution from the Ethics Committee.

However, that might not preclude the committee from beginning a probe, particularly if Campbell does formally ask for an investigation.

Even if Kontnik’s claim is accurate — that the $2,000 was a way of paying off her expenses — the method she chose to use would still raise many questions about ethical propriety, and Campbell’s knowledge of the payment could bring him under scrutiny by the panel.

Campbell’s press office did not return calls Monday seeking clarification on the Senator’s knowledge of the payback operation.

If the case were to turn into a criminal probe, it would most likely be handled by the Justice Department’s Office of Public Integrity or the U.S. Attorney’s Office in Denver, which would have jurisdiction because Thompson was employed in one of Campbell’s offices in suburban Denver.

Officials in the U.S. Attorney’s Office referred all questions Monday on Campbell to Justice Department headquarters in Washington, where officials remained mum.

One Justice aide noted the department’s “policy of not commenting on or even confirming the existence of an investigation.”

Senate records confirm an unusual spike in Thompson’s salary coming around the fall of 2002, the time he told The Denver Post that he received two bonuses in order to provide enough money to return $2,000 of it to Kontnik.

Thompson was hired on Nov. 15, 2001, as a staff assistant, according to expense reports filed with the Secretary of the Senate’s office. In a four-and-a-half-month span from his hiring until March 31, 2002, Campbell paid Thompson $7,040.

On May 16, 2002, Kontnik promoted Thompson to be her special assistant. In the six-month period ranging from April 1 to September 30, 2002 — the first bonus apparently came in September of that year — Thompson made $25,211, records show.

From Oct. 1, 2002, through March 31, 2003 — the second bonus payment apparently came in October — Thompson’s pay dropped back down to $18,939. He left Campbell’s office on May 8, 2003.

Whether the matter becomes a political issue remains to be seen.

“Any charge that is this serious deserves careful attention and consideration from the voters,” said Pete Maysmith, executive director of Colorado Common Cause.

But Michael Kanner, a political science professor at the University of Colorado, said that it is too early to tell whether the accusations of kickbacks on his staff could hurt Campbell politically. For now, he said, the news surrounding the sex scandal involving the university’s football team has been dominating local media coverage.

But for political insiders, Kontnik’s status in the Senator’s office had been a source of confusion and rumors for the past several weeks.

The Rocky Mountain News on Jan. 31 reported in a brief item that Kontnik was leaving. Four days later, the Post ran a story saying that Kontnik was giving up her chief of staff duties but would retain her pay and status and focus on appropriations issues.

Speculation on the matter did not abate, however, and Campbell’s foes sought to fan the flames by privately suggesting that the Senator was trying to suppress the news.

Greg Moore, editor of the Post, said earlier this month that while Campbell did not in any way try to spike a Post story on the staff shakeup, as one rumor had it, the Senator had called him to complain about the coverage of one Post reporter — the same reporter who broke the story on the kickback allegations.

Several days after Campbell’s call to the newspaper, his press secretary, Camden Hubbard, departed.

Campbell now faces what could turn out to be a tougher campaign than he anticipated without the services of his most loyal aide. When he switched parties in 1995 most of his Capitol Hill staff quit, but Kontnik stayed on.

“The whole episode raises a lot of questions about his management, about his leadership,” Woodhouse said. “There are a lot of questions about what’s going on. It’s got to be disruptive for his electoral efforts. It’s a story that isn’t going to go away today, and it’s going to dog him. For his office, it means a transition at best and chaos at worst.”

But Cinamon Watson, Campbell’s campaign manager, said the staff turmoil has no bearing on his re-election efforts.

“The Senator remains totally focused on the election,” she said. “He’s running aggressively.”

The confusion over Kontnik’s departure is hardly the first controversy surrounding Campbell or his staff. A former campaign manager and director of his state office, Sherrie Wolff, accused Campbell of harassment after they disagreed over whether she would continue working for him while she waged a campaign for Colorado secretary of state.

Campbell is also the target of an age discrimination suit filed by Rita Bastien, a former caseworker in his Englewood, Colo., district office.

When he served in the House, the Ethics Committee investigated whether it was proper for Campbell to sell jewelry he had made, and forced him to convert his jewelry business into a family partnership so it would not appear as if he were exploiting his status as a Member of Congress for profit. Opponents have also accused the Senator of casting votes that were beneficial to his family’s ranch in Colorado. In 2000, Campbell paid a $250 fine and performed 10 hours of community service after shooting at a neighbor’s dog near his home.

Despite these incidents, Campbell’s image as an avid motorcyclist, political independent and the lone American Indian in the Senate have resonated well with Colorado voters.

Still, Republicans cannot be happy about the prospect of running against Bridges, a deep-pockets donor to Democratic candidates and causes who runs a Denver-based think tank. Bridges, who made his fortune as a microbiologist, is expected to announce his intentions in the next several days. He did not respond to a message left at his Denver office Monday.

Democratic operatives said they believe Bridges is prepared to pour at least a few million dollars into the race, and will spend enough of his own fortune to remain financially competitive with Campbell. Through Dec. 31, 2003, the Senator had $1.3 million in his campaign account.

Watson cautioned Democrats for being prematurely euphoric.

“They don’t have a candidate yet,” she said.