Burnin' down the house in the Ky. Senate race

It's the most explosive ad of the 2014 campaign.

Showing an image of a house being lit on fire and going up in flames, Kentucky Democrat Alison Lundergan Grimes accuses Republican Sen. Mitch McConnell (R) of lighting the fire that led to the government shutdown.

"Mitch McConnell can't light the house on fire, then claim credit for putting it out," an announcer says in Grimes' first ad of her bid to unseat the Republican Senate leader next year.

The ad is evidence that one of the most important races of 2014 may just become the nastiest.

McConnell made a shrewd move in the aftermath of the government shutdown, which he helped broker an end to. He did several media interviews and claimed credit for his work to end the shutdown -- despite his Tea Party primary opponent.

"What happened yesterday completely steps on the whole rationale for her candidacy, which is that somehow I’m part of what she calls the dysfunction in Washington,” he said Oct. 18. "Look, I demonstrated on four occasions — including yesterday the most recent occasion, yesterday — that when the country is in crisis and something needs to be done on a bipartisan basis, I can step forward and get an agreement."

The ad points out that McConnell -- who has led the GOP strategy of making virtually every Democratic-pushed legislation need 60 votes to get through -- has called himself "a proud guardian of gridlock." McConnell also famously declared before the 2010 elections, "The single most important thing we want to achieve is for President Obama to be a one-term president."

The ad will get attention, and it indicates this is going to be a brutal race with two opponents who will pull no punches. It also shows, though, that McConnell's pivot has put Grimes on the defensive, which is usually the case when a candidate needs to use a highly provocative ad to make a point.