[T]hey can just sit on the sidelines lobbing tea bags until it becomes clear to the American people that no matter how bad the Republicans are, they can't possibly be as lame as the Democrats.
Obviously people make their choices for a variety of reasons, but I think some swing voters vote for Dems because they might do something for them and vote for Republicans because they know they won't do too much for someone else (especially the wrong kind of person, if you know what I mean). When Dems fail to deliver anything good, or means test the hell out of anything good, those voters quickly run back to the Republicans.