BRUNSWICK, Maine >> Former Gov. Angus King of Maine has a warning for the national political parties that may flood his state with negative ads to derail his independent candidacy for the Senate: “I will take note of how I am treated in this campaign.”

That is not an idle threat from some quixotic candidate for the seat being vacated by Maine’s famously moderate Republican, Olympia J. Snowe. It is a warning from the front-runner, a two-term independent governor who, because he refuses to say whom he would support for majority leader next year, may well decide which party controls an evenly divided Senate.

In a year of twists in the fight for Senate control, nothing has been as surprising as the sudden emergence of a campaign fight in Maine. Snowe was expected to cruise to re-election. Her dramatic announcement in February that she could no longer stomach the hyperpartisan atmosphere of Washington caught everyone flat-footed.

Initially, Democrats saw an unexpected opportunity for a clean pickup, which would be crucial to holding the Senate as they try to defend so many Democratic seats in Republican states. Then King scrambled the equation. His name recognition and popularity vaulted him ahead of the nearly dozen lesser-known candidates jockeying for the Republican and Democratic nominations in the June 12 primary.

It was not just Snowe’s timing that gave King an opening; it was also the way she bowed out, with her broadsides at both parties for what she considered failures of leadership and her decision to use her $2.4 million campaign war chest to rebuild “the sensible center,” not necessarily to aid the Republican nominee for her seat.

In an interview in Augusta last week, Snowe said the money would not go to King. “I don’t mean to say I haven’t had differences with my party, but I am a Republican,” she said.

Her actions, though, bolstered Mainers’ impression that the parties are broken, and that an independent may well be the antidote. A King campaign poll on Friday put his favorability in the state at 70 percent, against 16 percent who viewed him negatively. Eighty percent said they viewed his performance as governor, from 1995 to 2003, favorably.

“We’ve lost who we are,” said Mark Schoninger, 38, who greeted King at his store in Bath with “When you become senator,” no ifs about it.

“We weren’t founded by Republicans and Democrats,” Schoninger added. “We were founded by people who opposed tyranny, and we have these two parties who have become their own little tyrannies.”

Democrats seemed to sense that.

Maine’s two House members and King’s successor, John Baldacci, shied away from running. National Democrats pulled back and vowed to play nice with King. Their fear was a repeat of the 2010 race for governor, which a combative conservative, Paul R. LePage, won with 38 percent of the vote, ahead of the independent Eliot Cutler’s 36 percent and the Democrat Libby Mitchell’s 19 percent.

Republicans, on the other hand, are promising to keep the seat, archly accusing Democrats of striking a secret deal with King in return for his fealty.

Baldacci said any notion that King would run away with the seat ignored the outside money and negative advertising that Baldacci said was sure to flood Maine to keep the seat Republican.

“There’s going to be a storm coming through here in early November, late October that I don’t think people in Maine have ever seen,” he said.

King said he would be on guard for attacks from opponents who use outside groups that are nominally independent to deflect responsibility.

“The idea of an unaffiliated super PAC exists only in the fevered imagination of Nino Scalia,” he said, spitting out the nickname of Justice Antonin Scalia, who joined the Supreme Court’s Citizens United decision, the 2010 ruling that has set off a torrent of spending on negative campaign ads.

His opponents will have fodder: King left the state with a budget deficit before he headed off with his wife in an R.V., accepted a $102 million loan guarantee financed by a federal stimulus program to pay for a wind-power project and supports President Barack Obama’s health care law, in addition to not saying which party he would support to lead the Senate.

“People have the right to know,” Snowe said.

King said he was reaching out to parliamentary experts to see if he could demand Senate committee assignments without a choosing side, but he conceded that was not likely. To maximize Maine’s voice, he could simply side with the majority, he said. But there is a very real possibility he could determine that majority.

Which side King leans toward is not so obscure. He thinks the health care law was not ambitious enough. He would have voted for the stimulus and has no qualms about benefiting from it.

He will vote for Obama’s re-election, and he offers serious doubts about Mitt Romney, the likely Republican nominee.

“As a former governor, it bothers me he didn’t run for re-election” in Massachusetts, King said. “It bothers me I can’t get a fix on who he is.”

He opposes the prescription for Medicare in the House Republicans’ budget as “a recipe for a tremendous shift to the elderly of their health care costs.” And after a long conversation with Erskine B. Bowles, a chief of staff under President Bill Clinton, he said he was “dating” — but not marrying — the deficit-reduction plan put forward by Bowles and former Sen. Alan K. Simpson, a Republican. Taming the deficit without revenue increases “isn’t realistic,” he said.

Even so, King insisted he was independent. Giving an example of where he departs with Democrats, he said he might have voted against the Wall Street regulatory overhaul, saying it has caused too much collateral damage with community banks.

But he also pointed to an anti-smoking law he signed as governor. A cancer survivor, he pressed the Legislature to double the state’s tobacco tax. But to mitigate the impact on Maine’s working class, he proposed cutting 150,000 people from the state’s income tax rolls.

The Democratic-led Legislature went along with the tax increase but not the cut, with a bill that would spend the revenue on children’s health programs. Marian Wright Edelman, the children’s rights advocate, called King to beg for his signature. He vetoed that bill. The Legislature relented.

Last week, King brought a half-dozen fishermen into his campaign office to listen to their concerns. Gerry Cushman, 42, of Port Clyde, reminded him of an incident when he was governor. A fisherman told King that the population of striped bass was coming back, and that the fleet should be allowed to start hauling them in. King responded that as long as he was governor, no one would be fishing stripers in Maine.

His bluntness did not win him friends, but it won him votes, the fishermen agreed. “I disagreed with you, but I’ll vote for you,” said Bryan Bichrest, a 53-year-old fisherman from Cundys Harbor.

© 2012 The New York Times Company