As National Republicans have sought to chip into former independent governor Angus King's front-running status with a negative ad campaign, Senate Democrats' campaign arm will enter the fray for the first time with a six-figure ad buy set to begin Tuesday.

A Democratic Senatorial Campaign Committee strategist said the committee is spending $410,000 in Maine for 10 days of airtime beginning next week. It was not immediately clear what the focus of the ad would be.  But given that independent expenditure ads are typically negative commercials, there's a good chance the spot will hit Republican nominee Charlie Summers.

In a statement released Friday afternoon, DSCC Executive Director Guy Cecil slammed Summers. “Charlie Summers is an anti-choice Tea Partier who supports eliminating the Department of Education, privatizing Social Security, protecting tax breaks for companies that ship jobs overseas, and ending Medicare as we know it. Charlie Summers should not be in the United States Senate and it is time every Mainer knows it," he said.

As The Fix's Aaron Blake wrote Thursday, the Maine Senate race has become a tricky situation for national Democrats. Convinced that King would caucus with the Democrats if elected (even though he has not ever publicly said he would), party strategists have taken a hands-off approach and have all but ignored Democratic nominee Cynthia Dill.

Maine Democratic Party Chairman Ben Grant penned an op-ed in the Portland Press-Herald Friday expressing his support for Dill and questioning those who have opted to believe King will automatically caucus with the Democratic party if elected.

"He says he doesn't plan to caucus with anyone or that he hasn't decided. I'm choosing to believe that he's telling the truth, and that caucusing with the Republicans is a legitimate consideration for him. That alone is a disqualifier for me and many Democrats," Grant wrote.

The National Republican Senatorial Committee and other outside groups have spent $1.5 million boosting Summers, and recent polling in the race shows that it's having an effect, as the well-known former governor's once commanding lead has shrunk over Summers, who is running in second.

“It’s remarkable to see national Democrats now spending money in a state where they refuse to even endorse their own nominee," NRSC Executive Director Rob Jesmer said in a statement.

The focus of the DSCC spot will say a lot about the posture Democrats intend to adopt in the coming weeks. The launch of the buy is also an implicit acknowledgement that Summers is gaining in the polls.

Three-way races are difficult to read. Now-Gov. Paul LePage (R) won a slim victory in Maine in two years ago that also included an independent candidate.

Updated at 6:24 p.m.