Y'know, I think Markos and Jerome had something to say about this in their book...
From the Hill:
At least seven of the most vulnerable House GOP incumbents have been endorsed by unions, environmental activists or other Democratic-leaning advocacy groups. So have at least three of the most vulnerable Senate Republicans.
... Rep. George Miller (Calif.), ranking Democrat on the Education and the Workforce Committee, has disclosed that at least one of his House colleagues has said that, if Democrats fail to capture the House, labor will be partly to blame. Miller, a lawmaker close to House Minority Leader Nancy Pelosi (D-Calif.)...
Organized labor has also poured tens of thousands of dollars into the campaign accounts of highly vulnerable Republicans, in several instances surpassing the amount given to Democratic challengers.