Paid for, of course, by donations by the sorts of people who like three of the people in the ad. This reminds me of what Gray Davis did here in California some years back, when he highlighted the moderate aspects of Dick Riordan's record during the GOP primary in order to face a right-winger at the polls. It worked, but things didn't turn out so well for Gray even in the short run.
The cynicism of this sort of stuff, though...I recognize that politics sometimes requires people to make tactical arguments and compromises and such, but does anyone really think that this sort of stuff will make for anything more than a fleeting tactical victory on the way to losing the war? I'll tell you this, if the DGA ever sends me any fundraising materials I'll just print this out and send it back to them.