HELENA, Mont. (AP) — U.S. Sen. Max Baucus' surprise announcement that he won't run for a seventh term could mean a free-for-all for a Senate seat that has not been open since 1978, with a popular Democratic ex-governor and a freshman Republican congressman already showing interest.

Republicans said the open Montana seat helps their chances to gain six seats in 2014 to win a Senate majority. But many Democrats relish a potential early return to politics for former Montana Gov. Brian Schweitzer, who left office in January because of term limits.

Baucus told The Associated Press he made the decision not to run Monday night, even after he raised $5 million for a re-election fight that now won't occur.

He and his wife talked daily about the decision over several months, he said. They're building a new home in Bozeman and Baucus said he is looking forward to spending more time outside of politics.

"You have to do what you think is right for yourself and your family," Baucus said. "Life goes on. There is a time and place for everything."

Supporters lauded Baucus for a political career that spanned five decades, for playing a key role in some of the nation's biggest political debates and being the standard-bearer for the Montana Democratic Party through some very lean times.

But others — including some liberal detractors — sensed opportunity, which will likely lead to a boisterous battle to fill the power vacuum.

It didn't take long for Schweitzer's name to surface as a favorite for Democrats, while many Republicans advanced U.S. Rep. Steve Daines as their favorite.

Other possible GOP candidates include Denny Rehberg, the former congressman coming off a bruising and unsuccessful bid to unseat U.S. Sen. Jon Tester, and former Montana Gov. Marc Racicot, who served from 1993 to 2001 and later chaired the Republican National Committee.

Schweitzer, who previously said he's "not senile enough to be in the Senate," on Tuesday sounded as though he were open to the idea of replacing Baucus.

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"Some people see a pickup on the side of the road with a flat tire and say that's a problem. I'm the guy that stops and says, 'I'll fix it.' I like challenges. I like positive outcomes," Schweitzer told the AP.

If Schweitzer runs, he would likely be a shoo-in for the Democratic nomination and could even scare away Republican candidates, said Dave Parker, a political analyst at Montana State University.

"Schweitzer's in the catbird seat. It all begins and ends with him. He's the most popular politician in the state right now and if he runs he clears the field on both the Democratic side and the Republican side," said Parker.

Others said regardless of Schweitzer's decision, Republicans are bound to mount a determined campaign — and spend heavily — for the open seat.

"It's going to be another real race again. It's an open seat, so no matter what the race starts today," said Craig Wilson, a political analyst at the Montana State University, Billings.

University of Montana political analyst James Lopach predicted a Daines matchup against Schweitzer. He said Daines will find it hard to resist a Senate run — no matter the opponent.

"He's got to run statewide anyway. Why not run for a six-year position instead of a two-year position? The Senate is more prestigious and more powerful," Lopach said.

Daines' spokeswoman Alee Lockman said the new congressman is giving a run "serious and thoughtful consideration." His office released a statement saying the congressman appreciates Baucus' lifetime of service and that "it is critically important that this seat be filled by someone prepared to change the direction and culture of our nation."

Former U.S. Rep. Rick Hill, who lost to Steve Bullock in November's gubernatorial election, said he will be trying to convince Daines to run.

"He is a new leader with new ideas," Hill said.

Rehberg, who is now a co-chairman of the Washington-based public-strategy firm Mercury/Clark & Weinstock, did not return a call for comment Tuesday.

Two lesser-known Republicans already have announced their intentions to run — state Sen. Champ Edmunds of Missoula and former state Sen. and gubernatorial candidate Corey Stapleton.

On the Democratic side, EMILY's List President Stephanie Schriock and state Superintendent of Public Instruction Denise Juneau are possible contenders if Schweitzer decides not to run.

Baucus will be joined in retirement by fellow Democratic senators Jay Rockefeller of West Virginia, Frank Lautenberg of New Jersey, Tim Johnson of South Dakota, Tom Harkin of Iowa and Carl Levin of Michigan in announcing his retirement plans.

Republicans Saxby Chambliss of Georgia and Mike Johanns of Nebraska also have decided not to seek re-election next year.

Baucus in recent years has bemoaned the increased polarization in Washington but he said that was not a factor in his decision. Nor was his "no" vote on a gun-control compromise measure that would have expanded background checks, a vote that angered liberals, he said.

"It is just time to move on," Baucus said.

Baucus said he will focus his remaining time in office on tax reform, the farm bill, the North Fork Watershed Protection Act and his plan to expand land protections along the Rocky Mountain Front. Baucus said the health care overhaul that was a signature bill for him — one that cost him politically back home — remains important.

"I want to make sure health care is implemented, and implemented very well," he said.

That health care debate dinged his approval ratings, but Baucus believes the measure will eventually be popular. Baucus recalled a conversation he once had with former U.S. Sen. Mike Mansfield — a legendary Montana political figure. Baucus said when he was new to politics he was trying to tell Mansfield that the voters would reward them for "doing the right thing."

"He looked at me and just said, 'Yep, but sometime it takes a long time,'" Baucus said. "The bottom line is, it is important for me that Montanans know how grateful I am, and how humbled I am to have served Montana."

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Brown reported from Billings.