I'm sure the main reason Barack Obama's not going to campaign for Martha Coakley is that either a) the White House isn't that worried about her or b) they want to minimize the extent to which the President gets blamed if she does somehow manage to lose or only wins by a small margin.
What's interesting to note in the poll that we did on the race though is that Coakley is actually more popular than Obama- she has a 50% favorable rating while he has a 44% approval rating.
Now certainly you can make the argument that the reason for that is the conservative lean of likely voters at this point, and that if Obama showed up and changed the electorate he would have better approval numbers among those planning to go and vote.
But his visits in Virginia and New Jersey never did put any dent in the poll numbers- in fact Chris Christie and Bob McDonnell saw the polls move slightly in their direction after Obama's visits to their state. So there's an argument to be made that an Obama visit could actually have had more of a negative effect on Coakley than a positive one. But at this point it's academic.