Sen. Pete Domenici (R), the longest-serving Senator in New Mexico history, is expected to announce today that he will not seek a seventh term in 2008, multiple sources on and off Capitol Hill said late Wednesday.

Domenici is scheduled to make his announcement at 4 p.m. Mountain time at the St. Mary’s School in Albuquerque — where he attended grammar school and where his sister, Marianella Domenici, a nun, serves as principal.

Domenici, 75, has been in declining health in recent years and has been the subject of retirement rumors for several months.

Domenici’s departure would put another safe Republican Senate seat in play and could attract all three of New Mexico’s House Members and several others — including, possibly, Gov. Bill Richardson (D), who is running for president — into the race.

Those House races, in turn, could become even more competitive than they are now, potentially jeopardizing the GOP’s hold on two seats.

“It’s going to change the political scene in New Mexico the way it hasn’t for generations,” said Joe Monahan, an Albuquerque-based political consultant and blogger. “It’s just that big.”

Domenici has towered over politics in the Land of Enchantment like few other public figures. He served as Albuquerque mayor in the 1960s, ran unsuccessfully for governor in 1970 and was elected to the Senate in 1972. After one tough re-election race, he has glided to victory after victory, earning the nickname “St. Pete” in New Mexico political and government circles for his ability to spread federal largess across the state.

In the Senate, Domenici has been equally influential, serving as chairman or ranking member of both the Budget and Energy and Natural Resources committees for many years.

Domenici’s political sheen began to tarnish only in recent months, amid revelations that he called David Iglesias, then the U.S. attorney for New Mexico, to inquire this past fall about the status of politically charged federal investigations of Democrats in the midst of a difficult election season for the state GOP. Domenici has insisted that he did nothing wrong, but he is seen as having helped to persuade the Bush administration to oust Iglesias late last year, and Senate Democrats have tried to fan the controversy ever since.

Domenici will become the fifth Republican Senator so far planning to retire at the end of this term, joining Sens. Wayne Allard (Colo.), Larry Craig (Idaho), Chuck Hagel (Neb.) and John Warner (Va.) on the sidelines. Craig could wind up resigning before the end of his term due to the political fallout from his arrest this summer in an airport men’s room sex sting.

Allard’s, Warner’s and now Domenici’s departures have instantly transformed those races to tossup status, and the Nebraska Senate election will be tight if former Sen. Bob Kerrey (D) decides to run.

Spokesmen for the National Republican Senatorial Committee and the Democratic Senatorial Campaign Committee remained mum late Wednesday, but it is clear that New Mexico will become a major Senate battleground in 2008 — and a prime pickup opportunity for surging Democrats.

Senate sources said that if he hadn’t begun calling him already on Wednesday, DSCC Chairman Charles Schumer (N.Y.) is certain to reach out to Richardson — whose long House term overlapped with Schumer’s — soon and will urge him to run. Richardson, who won a second term as governor last year with 69 percent of the vote, is widely seen as the strongest candidate the Democrats could run.

But Richardson has said for months that he has no interest in returning to Congress, and a campaign spokesman said Wednesday evening that Domenici’s retirement plans do not change the governor’s resolve to remain in the White House race.

“We are focused on winning the nomination to be president,” spokesman Tom Reynolds said.

Still, some Democrats hold out hope that Richardson can be persuaded to run for Senate, especially with Domenici leaving. The filing deadline in New Mexico is in mid-February, just days after the Feb. 5 “Tsunami Tuesday” presidential primaries, when voters in 20 states will go to the polls and the presidential nomination fights may end.

“All eyes will be on Big Bill and will look to him to give some kind of signal” about his plans, Monahan predicted.

Other potential Democratic candidates include Rep. Tom Udall, who won two statewide races for attorney general, Lt. Gov. Diane Denish, Albuquerque Mayor Martin Chavez and former state Attorney General Patricia Madrid.

Denish and Chavez already have announced their intentions to run for governor in 2010, when Richardson is term-limited, but one may be persuaded to change course if Richardson does not enter the Senate race.

Currently, Santa Fe real estate developer Don Wiviott is the leading Democratic Senate candidate, and though he recently seeded the campaign with $400,000 of his own money, he is almost certain to be overtaken by one of the better-known candidates.

On the Republican side, Reps. Heather Wilson and Steve Pearce both are expected to look closely at the race, and it is conceivable that both could run. While some New Mexico insiders believe that Wilson, a political moderate, could be a stronger candidate in the general election, Pearce, who is more conservative, may have the upper hand in a GOP primary.

Wilson, a perennial Democratic target in her marginal Albuquerque district, also has been singed by the fallout from the Iglesias affair and could be weakened politically. But she is a prodigious fundraiser who frequently defies the odds in her re-election races.

If Wilson or Pearce runs for the Senate, either’s district could become even more competitive than it is now. Democrats already are high on Albuquerque City Councilor Martin Heinrich, who is seeking to unseat Wilson, and Republicans have no obvious strong heirs to the Congresswoman.

In the more conservative 2nd district, Doña Ana County Commissioner Bill McCamley is the leading Democratic challenger to Pearce, though the race could change if the Congressman runs for Senate. Republicans likely would have several candidates looking to enter the House race if Pearce sought the Senate seat.

None of the would-be Senate candidates had said anything publicly about the race as of press time Wednesday evening.

David M. Drucker, Erin P. Billings and John Stanton contributed to this report.