AUGUSTA — The floodgates opened Monday and through them poured a tidal wave of candidates for Maine’s two congressional seats.

The cascade of candidates was precipitated by the announcement that 16-year Rep. Olympia J. Snowe would move up to campaign for outgoing Sen. George J. Mitchell’s U.S. Senate seat, and the expected announcement today by Rep. Thomas H. Andrews that he would square off against Snowe.

Mitchell’s surprise retirement and the Snowe-Andrews Senate battle means that the two U.S. House seats in Maine’s 1st and 2nd districts will be up for grabs.

Once Snowe had made her intentions clear Monday, state Rep. Stephen M. Zirnkilton, R-Mount Desert, who had been opposing Mitchell in the Senate race, stepped aside and said he’d now run for Snowe’s 2nd District seat in the House.

In the 1st District, Maine Senate President Dennis L. “Duke” Dutremble, D-Biddeford, after conferring at length with Andrews, announced he was running for Andrews’ 1st District seat.

On whether Andrews will announce today that he’s going to campaign for the Senate, Dutremble said, “I talked to Tom and he made it clear he was going to run for that position.”

Last week, Dutremble had sounded like he might go for the U.S. Senate seat.

“I did a lot of soul-searching and I felt it would be best to run for the 1st District,” Dutremble said. “I do have a lot to sacrifice, but we have lost a tremendous leader in Washington and I feel I can go there and replace some of that strong work for Maine people.”

Dutremble has served as Senate president about 15 months, having succeeded Sen. Charles Pray, D-Millinocket, in that position. Because of state term limits, Dutremble could serve only another two years in the Senate.

“What you have in a situation like this is people seeing an opportunity arise and they have to decide what the right thing for them to do is,” Dutremble said. “I’ve been in politics since I was 8 years old. Politics is in my family. Right now I think I can do best for the 1st District by running for that seat.”

Senate Majority Leader Donald Esty, D-Westbrook, said he’d ruled out a campaign for the 1st District seat, but instead would try to become Dutremble’s successor as Senate president if he’s re-elected.

“I think now more than ever it is important that someone stay in the Maine Senate,” Esty joked.

Zirnkilton, the 35-year-old assistant Republican leader of the House, had been running in a Republican Senate primary against Glenn MacNaughton of Greenville to see which one would challenge the heavily favored Mitchell. MacNaughton said Monday he was going to stay in the Senate race, facing Snowe instead of Zirnkilton in the Republican primary.

Zirnkilton said he was in excellent shape to convert his U.S. Senate campaign to a campaign for the 2nd District congressional seat.

“I have been campaigning for five months,” he said. “My desire to represent Maine people in Washington is no less than it was before Sen. Mitchell resigned. I have a staff in place. I’ve raised funds. I feel very prepared and anxious to move forward.”

In the 2nd District, Zirnkilton may be challenged on the Republican side by his friend, Rep. Richard Bennett, R-Norway, who had considered and then dropped the idea of running against Mitchell in the Senate race.

The Democratic primary looks much more crowded.

Rep. Mary Cathcart, D-Orono, and Jean Hay of Blue Hill have filed papers and are in the race. David Costello of Lewiston and Shawn Hallisey of Machiasport also have said they intend to run.

James Howaniec of Lewiston, a Democratic candidate for governor, said he was seriously considering getting into the congres- sional race. James Mitchell of Bangor, the Maine Democratic Party chairman, also was seriously considering a congressional run.

Robert Woodbury said over the weekend he definitely would continue his campaign for governor.

Another potential Democratic candidate in the 1st District is Janet Mills, district attorney for Androscoggin, Oxford and Franklin counties.

“I’m very interested in it,” Mills said.

Jean Hay said, “I was ready to take on Olympia. I am in some ways disappointed, but it certainly puts me in a better position. I’m excited about it. The adrenalin is pumping. I’m just delighted.”

Costello, a former top aide to Secretary of State G. William Diamond, said he would decide whether to run based on who enters the Democratic primary.

“I’ve got to see how it shakes out,” he said. “I’ve got to see who gets in.”

Howaniec, the former Lewiston mayor, said, “We’re about 24 hours away from a final decision. We’re going to sleep on it one more night. We’re taking an extremely close look at running in the 2nd Congressional District. I think I could be a very viable candidate.”

The campaign in the 1st District also was overflowing with candidates and would-be candidates.

On the Democratic side, Dutremble could be challenged by Sen. Gerard Conley Jr. of Portland. Others said to be considering a Democratic congressional run were Augusta Mayor William Burney, Sens. Dale McCormick of Monmouth, Bonnie Titcomb of Casco and Mark Lawrence of Kittery, and Rep. Sharon Treat of Gardiner.

On the Republican side, Sen. Charles Summers of Scarborough began circulating nominating papers for the 1st District race. James B. Longley Jr., son of the late independent governor, said previously he intends to run, but York County Sheriff Michael Bemis said Monday he has decided not to run.

Kevin Keogh, chairman of the Maine Republican Party, and Abigail Holman, an aide to Gov. John R. McKernan, also were possible candidates in the 1st District.