Vulnerable Democratic Sen. Claire McCaskill, battling for re-election in Missouri, released a new TV ad today knocking third-party groups who attack her. The spot also seeks to reinforce her image as a fighter for average folks.

Since McCaskill does not yet have a GOP opponent — businessman John Brunner, Rep. Todd Akin and former state Treasurer Sarah Steelman are all locked in a primary battle — her strategy appears to be to go on the offensive against third-party groups.

“They just keep coming back,” a male narrator says in the spot, to visuals of TVs on an assembly line with anti-McCaskill ads on them. “Secret money, attacking Claire McCaskill. These big oil and insurance companies don’t want you to know who they are, but here’s what they want to do.”

https://www.youtube.com/watch?v=NqSX4AJ1_J0

The ad is backed by a $300,000 buy statewide.

McCaskill, who was an early and loud supporter of President Barack Obama and his agenda, faces a particular challenge in a presidential election year in a state that is almost certain to vote for presumptive GOP White House nominee Mitt Romney.

Roll Call rates the race as a Tossup.