The conservative 501(c)(4) nonprofit Crossroads GPS is hitting the airwaves for the first time in the Maine Senate race, with a spot targeting independent candidate Angus King.

The airwaves have grown increasingly crowded in recent weeks, as King's onetime firm grasp on the three-way race appears to have loosened. The Crossroads ad exclusively targets the former governor. 

"King supported big tax increases that would have hurt small businesses. Worse, as governor, King slashed funding for our schools. And King increased spending by 51 percent," says the narrator of the 30-second spot.

Both the National Republican Senatorial Committee and the Democratic Senatorial Campaign Committee have joined the air war in Maine. Crossraods's entrance into the fray is the latest sign that Republicans still think GOP nominee Charlie Summers can win. 

King, the popular former governor, remains the front-runner in the race, polls show. But the NRSC attack ads took enough of a toll to prompt the DSCC to join the fight. 

Officially, the DSCC is neutral in the race. But most national Democrats privately believe that King would caucus with the party if he is elected. Democratic nominee Cynthia Dill hasn't received any national assistance. A DSCC spokesman responded to the Crossroads ad on Tuesday by bashing Crossroads co-founder Karl Rove, but he didn't mention any of the candidates in the race. 

“Karl Rove is spending millions propping up Republican Senate hopefuls because he knows that these candidates would be loyal rubber stamps for Rove’s special interest agenda," said DSCC spokesman Shripal Shah. 

The Crossroads GPS buy in Maine is part of a larger seven-state, $5 million buy that also includes commercials in Indiana, Montana, Nevada, North Dakota, Virginia and Wisconsin (the Wisconsin spot will be released on Wednesday). 

Updated at 6:26 p.m.