Well if Romney is the nominee he will at least have an excellent chance of getting over 40% of the vote, so that might be enough to prevent any Obama coattails. He seems liberal enough to win the district in a presidential year.
In fact that's the only scenario under which he (Tisei) can win. Almost any other Republican presidential candidate will lose badly in Massachusetts, so coattails will be strong in these cases. I would likeTisei to win both
because i am not impressed by Tierney with his problems, and even more - because i am a heavy fan of "mavericks", and now that usually means (reltively) liberal Republicans and (relatively) conservative Democrats)))