* Poll shows King could win three-way race
* Snowe had been expected to breeze to re-election
By Ros Krasny
March 15 (Reuters) - A decision by big-name Democrats to pass up Maine's U.S. Senate race and the candidacy of a prominent independent have shaken up a contest that once looked like a potential Democratic pickup after the retirement of moderate Republican Olympia Snowe.
Snowe's decision in February not to seek re-election set off a scramble among potential candidates and complicated the Republican effort to wrest control of the Senate from Democrats in November's elections.
Angus King, a former governor, announced on March 5 he planned to run for the seat as an independent. A recent poll showed that King, who was governor from 1995 to 2003, could win a three-way race.
Party-affiliated candidates had until 5 p.m. local time on Thursday to present the minimum of 2,000 signatures needed to compete in June primaries. Six Republicans and three Democrats did so, according to the Maine Secretary of State's office. Most of the candidates are current or former state officials or lawmakers.
The Northeastern state's two Democratic U.S. representatives, Chellie Pingree and Mike Michaud, had begun collecting signatures to run, but later changed their minds. Also declining to run was former two-term Democratic Governor John Baldacci.
None of the Republicans running approaches the political stature or name recognition of Snowe, who was expected to breeze to a fourth six-year term despite worries among some party members she was not conservative enough.
Maine voted for Democrat Barack Obama in the 2008 presidential election by an 18-point margin. The state's other U.S. senator, Susan Collins, is also a moderate.
In announcing her decision to step down, Snowe rapped the "atmosphere of polarization" in Congress and wondered whether much could be achieved with a new term.
It is unclear whether King, 67, known as a fiscal conservative and social moderate, would caucus with Democrats if elected senator. Democrats now hold a 53-47 U.S. Senate majority, which includes two independents who caucus with them, Joe Lieberman of Connecticut and Bernie Sanders of Vermont.
King has been noncommittal, but the National Republican Senatorial Committee posted an ad on YouTube implying he was a shadow Democrat pulled into the race by party leaders, specifically Senator Chuck Schumer, as an alternative to more liberal Democrats like Pingree.
"Maine deserves better than smoke-filled backroom deals," the ad's narrator says.
As governor, King worked at times with a Democratic-controlled House and a Republican Senate. He supported Obama in 2008 and has backed the president for re-election - but also favored Republican George W. Bush in 2000.
Public Policy Polling showed in early March that King might win a three-way Senate race among a variety of Democratic and Republican challengers. The same survey showed King with a 62 percent favorable rating, well above others in the race.
Maine Democrats have painful memories of how an independent candidate can shake up a race. In 2010, Paul LePage, a conservative Republican, was elected governor with 38 percent of the vote when he narrowly beat Eliot Cutler, an independent, pushing the Democrat to a distant third.
(Reporting By Ros Krasny in Boston; Editing by Paul Thomasch and Peter Cooney)