The National Republican Senatorial Committee bought almost $600,000 in television time here, believing that popular former Gov. Angus King (I) is more vulnerable than the available polling suggests.

King is the frontrunner and is expected to caucus with the Democrats if he wins the Senate seat in November. But in a complex three-way race with Maine Secretary of State Charlie Summers (R) and state Sen. Cynthia Dill, the NRSC senses opportunity. The Democratic Senatorial Campaign Committee has not invested in Dill, but Republicans say that if her numbers can be elevated, she and King might divide the vote of Democrats and Democratic-leaning independents, allowing Summers to win with a plurality.

The NRSC ad buy, which is built around that strategy, is scheduled to last two weeks, according to sources who monitor Maine ad spending. The news of the NRSC offensive was first reported by the Hotline.

National Democrats completely dismiss the NRSC’s Maine strategy, arguing that the GOP is on its heels in Senate races.

“The map continues to move decisively towards Democrats,” DSCC spokesman Matt Canter said. “National Republicans have all but abandoned their candidates in New Mexico and Hawaii, and they’re now playing defense in Maine, a state that they once claimed solidly in their column.”

This is an open-seat race thanks to the retirement of Republican Sen. Olympia Snowe. Roll Call rates this race Likely Independent.