I seriously doubt that having a liberal Republican as our presidential nominee will have a "coattail effect" on congressional races. If anything, it will cause some conservatives to stay at home, which will result in fewer votes for congressional Republicans. A liberal Republican as our presidential nominee will also cause more RINOs to go and vote, and in most cases such RINOs will vote for the Democrat congressional candidate rather than for a conservative Republican.
BTW, of the "big three," only Giuliani is clearly a liberal RINO. Romney used to take liberal positions on which he has backed away of late, so we don't know if he'll attract RINO voters and drive away conservatives. As for McCain, he's a "conservative RINO," someone who appears to be hell-bent on hurting the GOP in order to promote himself yet who takes conservative positions on nearly all issues.