The Chamber, Crossroads and its sister group Crossroads GPS have spent hundreds of thousands of dollars already against McCaskill. Her response is significant: a $200,000 statewide ad buy, according to a Republican ad tracker.
The ad shows the changing dynamics super-PACs and other big-spending outside groups have created in some races. McCaskill has been hammered on TV and radio by these groups, who have been as prominent in attacking her as her actual opponents. Her response indicates she sees the attacks as a threat.
She'd already responded this week by airing radio ads counter-attacking against a Crossroads radio ad buy on the issue of President Obama's requirement that all insurance plans cover contraception.
McCaskill likely faces a tough reelection fight in the Republican-trending state, although her GOP foes have so far failed to impress. Aside from the three in the race, Missouri state Auditor Tom Schweich (R) is also mulling a bid.