Here's something to consider when talking about Barack Obama and the results of today's elections.
In New Jersey only 75% of voters who approve of Obama's job performance plan to vote for Jon Corzine. And in NY-23 only 71% of people who think Obama's doing a good job plan to vote for Bill Owens.
I guess you could claim that shows a failure on Obama's part to convince his backers to vote for folks he supports. But I think it's more an indication of Owens and Corzine being flawed candidates. A significant number of voters in those races are voting for Republicans or independents not because they're unhappy with Obama but because they simply think the Democratic candidates aren't very good. He's done his job with those voters by making them happy with his performance as President but that's only going to go so far in convincing them to vote for candidates they don't like.
And in New Jersey it should be kept in mind that Corzine's approval/popularity was in the 30s in some polls as far back as February and March when Obama's standing was at its peak...it's not as if his standing has declined in conjunction with Obama's.
All that said, today's elections do have a lot to do with Obama because his tenure in office has made Republicans a lot angrier than it has made Democrats happy, and that's likely to be reflected in today's turnout patterns. That's not Obama's fault per se, but it's certainly something Democrats are going to have to deal with to avoid a catastrophic 2010 election. Nailing down some solid accomplishments, like passing health care, should get the Democratic base a little more energized about coming out next year to vote to continue the progress. But that won't save them today.