Former Maine governor Angus King speaks in March at Bowdoin College in Brunswick. King announced plans to run as an independent for the seat being vacated by Sen. Olympia Snowe (R). He says the Senate needs an independent voice that's not beholden to Republicans or Democrats. (Joel Page/AP)

Angus King, a popular former Maine governor and the favorite to become the state’s next U.S. senator, thinks the way to win an election in 2012 is to stake out the middle ground, crusade against partisanship and present himself as a devout independent.

It is a bold strategy in this hyperpartisan age, and the depths of his moderation are captured by two photographs positioned side by side at his campaign headquarters: one of Ronald Reagan, the other of Robert F. Kennedy.

Maine voters know that King voted for George W. Bush in 2000 and plans to vote for President Obama in 2012. What they don’t know is whether their independent senator would align himself more with Republicans or with Democrats, and that has added an overlay of Washington intrigue to the contest.

“My desire is to be as independent as I can be, as long as I can be, subject to being effective,” King said. “I’m not going just for symbolism. I want to do something.”

First, he must win, and he’s betting that an appeal to the even-tempered middle can trump the partisan passions that animate the extremes in both parties.

“I’m more convinced than when I announced that I’m on the right track,” King said. “Everywhere I go in Maine . . . it’s all they want to talk about. They want me to go down there and talk some sense into those people — go down there and make it work.”

With the balance of power in the Senate decided by a close margin, King’s unwillingness to commit to one side or the other has scrambled the calculus in Washington and brought him a lot of attention.

“I’ve come to realize that an unencumbered U.S. senator is a profound threat to the whole system,” he said. “It’s somebody that they can’t put in a box and say, ‘Oh, well, we know how this guy is going to vote.’ That has raised the stakes, frankly.”

And the stakes were already pretty high.

Leading the race

On Tuesday, six Republicans and four Democrats will face off in Maine’s Senate primaries. But polls show that King, who as an independent won’t be on Tuesday’s ballot, has a wide lead over any of his potential challengers in the general-election race to succeed the retiring Sen. Olympia J. Snowe (R).

Democrats control the Senate on the strength of a slim three-seat majority, and Republicans have a good chance of picking up the four seats they need to take control of the chamber. If that happens, and if the GOP can hang on to its majority in the House, it would significantly alter the political landscape in Washington for a reelected Obama or a newly elected Mitt Romney.

Snowe upended the Republican takeover plans in February with her decision not to seek reelection, and her reasons for leaving the Senate converge with King’s rationale for trying to replace her. Snowe cited “an atmosphere of polarization and ‘my way or the highway’ ideologies.”

After her decision, King, 68, quickly jumped into the race and rocketed to the top of the polls, fueled by his name recognition, high approval ratings and anti-partisan message.

Snowe clearly admires King’s strategy. “I think that people have to reward those individuals who are prepared to work across the political aisle,” she said last week. “I don’t see any other way; if you don’t talk to people with whom you disagree, you’re never going to solve problems.”

A secret Democrat?

The two parties have approached King’s candidacy in different ways.

Republicans think he is a Democrat masquerading as an independent, while Democrats are quietly hoping that the GOP is right. The National Republican Senatorial Committee charged in a Web video that King was “dragged into the race” by Democrats, who “shove[d] aside more liberal candidates.” The NRSC and outside super PACs are expected to spend millions to try to prevent a King victory.

Democratic officials say privately that they are unlikely to invest much in the winner of Tuesday’s Democratic primary, in the hope that King will eventually support their party.

King said he has never spoken to Senate Democratic leader Harry M. Reid (Nev.) or Republican leader Mitch McConnell (Ky.), but he understands that he may eventually have to choose between them.

“I don’t want to stand in the middle of the aisle and say I’m an independent and not have a committee assignment,” he said. “That’s sort of self-defeating, and it wouldn’t be fair to Maine. If it’s necessary to join a caucus and get a committee assignment, I’ll do it.”

It would depend on the terms, however. “What does ‘join a caucus’ mean? Does it mean casting one vote to organize the Senate, and then you’re on your own? Or does it mean you have to truly join the caucus, go to the meetings and participate fully, or you lose your committee assignments?” he said. “How the parties handle that with me is going to have a significant influence on my decision.”

King — who as governor claimed no party allegiance — is part of a long tradition in Maine and New England of political moderation and independence, a legacy that has included such senators as Republican Margaret Chase Smith, Democrat George Mitchell and Snowe. But that tradition is now under siege, and King has not run a political campaign in the 21st century.

Regardless, his centrist message should have broad appeal, said Amy Fried, a political science professor at the University of Maine.

“Some claim that King was never really tested and that perhaps he could go down if he’s hit by both sides,” Fried said. “But Maine has certain elements of its political culture where I think that would be very difficult. Once a candidate gets to a certain status, where they’re generally really respected, then it’s really hard to do any negative attacks against them. Voters dislike that kind of negative activity and take it personally.”

Candidates’ criticism

State Treasurer Bruce Poliquin, a leading GOP candidate, said in an interview that King would be “the second Democrat in the race.” Another Republican contender, Charlie Summers, said the former governor’s insistence on neutrality shows “how detached King is from the people he wants to represent.”

Top Democratic candidate Matt Dunlap doubts that King could be a Senate bridge-builder: “If President Obama couldn’t bring the two sides together, and Olympia Snowe couldn’t do it and Joe Lieberman couldn’t do it, I don’t know why Angus King can.”

Cynthia Dill, another Democrat in the race, said King is playing “mind games” with voters and conducting “a social experiment.”

King “either knows who he’s going to caucus with and he’s not telling us — in which case that’s not the kind of change we’re looking for in Washington, another politician who’s not really straight with voters — or he doesn’t know,” Dill said. “And it concerns me if he doesn’t know.”

Already King has changed positions on the George W. Bush-era tax cuts, set to expire Jan. 1. Early in his campaign, he said that tax cuts for upper-income Americans should end. But now, after last month’s poor employment report, he says that decisions on taxes and spending should be tied more closely to economic indicators instead of expiration dates. He said he supports most of the proposals from the bipartisan Simpson-Bowles fiscal commission and is studying other issues.

But he’d much rather talk about fixing the Senate.

“If you have a series of leaks in the pipes in your house and your wrench is broken, you never get to the leaks,” he said. “Talking about this unfunctionality of Congress is not an academic exercise. It’s fundamental to then get to a place where we can get to these problems.”

As for his potential opponents, King would offer only that “they’re all nice people — I assume.”

Since his last statewide campaign, in 1998, he said, he’s most impressed by how social-media tools are transforming campaigns. A devoted Facebook user, he spends at least an hour each night on the site answering voters’ queries.

King never faced the wrath of super PACs during his previous campaigns, and he expects that outside groups will spend roughly $5 million attacking him — in a state with just 1.3 million residents.

“It’s going to be fun most of the time, but it’s not going to be fun to be shot at,” he said of the campaign. But to get a chance to fix the Senate, “I’m willing to put up with a little aggravation.”

Polling analyst Scott Clement contributed to this report.