A few pundits and even the McCain campaign to some extent have been presenting the argument that one reason people should vote for McCain is because all indicators point to Democrats not just keeping control of Congress but expanding their majorities. If Obama is elected president, then we'll have one party rule which was bad under President Bush and the Republicans. Forgetting the intellectual dishonesty here (is McCain not going to campaign for a Republican congressional majority in 2010 should he be elected?), is this really a good argument in our current times?
Many Americans tend to like divided government in general, but it would mean 4 years of gridlock. Facing such big issues, Americans want our government to do something about them. Obama could implement his policy ideas because he'll have a Democratic Congress to back him. For McCain, does anyone thinks his health care plan will pass? His massive tax cuts for the rich? All of his ideas would at the very least by altered by Congress, and thus his campaign promises are much less likely to be realized than Obama's. The bottom line is he is much less likely to be effective at governing than Obama given the political realities, so the Obama people seem to have a great counterargument here.