Speaking to the The New York Times, Rob Jesmer, head of the National Republican Senatorial Committee, makes a good point about efforts by congressional Democrats to distance themselves from President Obama:
"Whether they get on stage with him is not going to matter," said Rob Jesmer, executive director of the National Republican Senatorial Committee. "They are going to rise or fall with him in most cases. The reason he is unpopular is because of his policies and these are the policies they have voted for – health care, the stimulus, now a second stimulus. There isn't any daylight in the minds of voters." [Emphasis mine]
It's worth taking a look at this chart on split-ticket voting from political scientist Alan Abramowitz, plotted using data from the American National Election Studies:
Increased polarization over the last three decades has produced a sharp decline in split-ticket voting, which means – in other words – that Jesmer is right; voters are more likely to evaluate each party in an election, rather than any individual candidate. Overall next year, the fortunes of congressional Democrats will rise and fall with those of President Obama. If Obama wins reelection, then it's likely that Democrats have done well. If he's on the way to a loss, then there's little they can do to stem the tide.