Eight days after Sen. Olympia Snowe (R-Maine) turned the political world on its head with her retirement announcement, the succession battle has kept heads spinning.

The latest kink in both national parties’ plans was Rep. Chellie Pingree’s (D) Wednesday announcement that she would not seek the seat. That left Democratic, Republican and even Independent operatives scrambling.

At the center of all the drama is former Gov. Angus King (I), who announced Monday that he was launching his bid for the Senate. He is a serious contender and now the frontrunner for the seat. Speculation abounds about which political party he will chose to caucus with and how a possible Independent Senator could affect the balance of power in the Senate in 2013.

Democrats repeatedly said Pingree was their strongest potential candidate, but in her statement Wednesday, she said, “I concluded that I will best serve the people of Maine by running for re-election to the House.”

Former Gov. John Baldacci (D) and former Maine Secretary of State Matt Dunlap (D) are also in the early stages of building campaigns.

While the GOP field is also in flux, national Republicans spent most of the week fueling a narrative that Democrats pushed Pingree out of the race and that Senate Democratic leadership was secretly negotiating with King to join the caucus if elected.

“That is completely untrue,” said a source familiar with Pingree’s thinking. “I never saw any attempt to push her out.”

But the question of whom to caucus with remains a big one for King. Because of his unique brand and popularity, speculation was rampant, but his nascent campaign refused to disclose a preference.

“We have discussed it,” King spokeswoman Crystal Canney said. “He will caucus where it will be most effective.”

When asked whether voters will know any sort of inclination prior to the election, she said it was “very early in the campaign” to come to such a conclusion.

One Democratic insider said King’s entry into the race is a “big get” for the Democrats and their effort to hold the majority. National Republican Senatorial Committee Chairman John Cornyn (Texas) said he fully expects King to caucus with the Democrats if elected, based on discussions with Sen. Susan Collins (R-Maine). But Cornyn made it clear he is not conceding the race and is hoping to help snag a formidable GOP challenger.

Senate Democratic leaders were not forthcoming about the extent of their involvement, if any, in recruiting King to run or securing any commitment from him to caucus with their party. But Democratic operatives and top Republicans said they expect King to join the Democratic Conference if he is elected in November. Before King became an Independent, he identified as a Democrat.

NRSC Executive Director Rob Jesmer accused Democrats of cutting a “backroom deal” with King, but Canney strongly denied the accusation.

“There’s absolutely no deal,” Canney said. “I couldn’t be more firm about that.”

Only a week ago, Pingree’s candidacy seemed almost a sure bet.

“Chellie Pingree saw polling that said she was going to have a very hard time beating King and could instead hand the race to the GOP, so she made the decision on her own to drop out,” a Democratic strategist said.

“There’s a chance she could have beat him, but she did not want to lose her House seat,” one state Democratic operative said.

But the Senate math is what weighed heavily on the party, and Maine Democrats are haunted by the 2010 three-way gubernatorial race. A Democratic candidate and an Independent candidate split voters, and tea-party-aligned Gov. Paul LePage emerged as the winner.

Pingree was a favorite with the liberal base, and the reaction from interest groups and state Democrats was generally positive.

Adam Green of the Progressive Change Campaign Committee issued a statement acknowledging the political reality of the race.

“Angus King was willing to create a three-way race that handed this Senate seat to a far-right Republican,” he said. “Chellie Pingree’s decision was based on what is best for the people of Maine and the future of progressive causes, not based on what was best for herself.”

Despite his popularity, King still faces obstacles as an Independent. Political strategists question how much money he can personally commit to the race and how an Independent can win a Senate race without a national party apparatus behind him.

But one unaligned state political strategist brushed off the financial considerations.

“With Chellie out, the price tag of this race just went down $3 [million] to $6 million,” he said.

As for the Republican side, three major contenders have taken the first steps toward launching campaigns: Maine Secretary of State Charlie Summers, state Treasurer Bruce Poliquin and state Attorney General William Schneider.

Schneider rented out Snowe’s email list and sought petition signatures, and one other campaign has inquired about doing the same.

“We’re talking to several candidates because we were scrambling to try to make sure that we could get one or more candidates to qualify for the ballot. That’s sort of where we are now,” Cornyn said. “It’s getting kind of curious. The most likely candidate to run as a Democrat has now folded her tent and is not running.”

Both parties also expressed concern about the impending filing deadline of March 15. The compact timeline between Snowe’s announcement and the deadline has led some to seriously wonder whether it is realistic for all of the serious candidates to obtain the 2,000 petitions needed to get on the primary ballot.

King has an extended deadline because he is not seeking a party nomination.

David M. Drucker contributed to this report.