On Wednesday, we'll begin talking about whether whoever gets elected has a "mandate." We'll talk about it even more if Barack Obama is re-elected, because when a new president takes office we accept that he'll be doing all kinds of new things, changing course on almost every policy, replacing all the members of the other party who populate the executive branch with members of his own party, etc. With a re-elected president, on the other hand, there's a real question about where he goes from here and how much he can try to accomplish. There's a fundamental problem with the mandate idea, however, that makes it almost meaningless in today's Washington.
The mandate notion assumes that the larger the president's margin of victory, the greater the proportion of the public has signed on to his policy agenda. That's not completely unreasonable, though in practice most voters have only the vaguest notion of what the person they're voting for wants to do. But the idea of the mandate is about Congress more than anything else. There's a chain of responsibility: The public gives the president its nod; he puts his agenda forth in the form of executive actions, appointments, and legislation; and Congress approves those actions because the public has said with its presidential vote that it wants them. If Congress stands in the way of a president who won a mandate, then the public will rise up and punish them, while if they stand in the way of a president who didn't win as much of a mandate (because he won without a popular vote majority, for instance), then the public will approve.
You can see the problem in this logic. For this chain to operate, members of Congress have to be either temperamentally inclined to go along with whatever they perceive as the broad public will, or forced to do so because they fear the political consequences. But if Obama wins and is left with a Republican House, he'll be facing members of Congress who don't really care what the public thinks or whom it allegedly gave a mandate to.
Although a few of the nuttiest Tea Partiers may lose their seats on Tuesday, we're going to be looking at a Republican caucus pretty much the same as it is now. The two most important things to know about them are that 1) they are true believers, and 2) they're mostly elected in safe conservative districts, so they don't fear retribution at the polls for being obstructionist.
When I say they're true believers, I mean not only that they have their own extremely conservative agenda, but also that many of them feel that Obama is an illegitimate president whose agenda will send America tumbling toward a nightmarish socialist dystopia. They see implacable opposition to anything and everything Obama wants to do as a moral obligation. To them, it matters not a whit whether he wins by one vote or by 20 million votes. Their behavior will be the same either way.
That isn't to say there aren't also people within the Republican party in general and in the House in particular who have a firmer grip on reality. Speaker John Boehner is one of them; he knows that their reputation as mindless obstructionists has done his party real harm, and if he had the power to dictate his caucus' actions he would probably have them dial the opposition back a bit and find ways to look more cooperative without giving away too much. But he doesn't have that power. Every time he needs to get their votes on something important it's a struggle. Many House Republicans would be happy to see him go. His second-in-command, Eric Cantor, is just waiting for the right opportunity to plunge a knife in Boehner's back and take his job.
So I can guarantee you that no matter what the specific margin is, if Obama wins on Tuesday, Republicans will act as though he has no mandate. They'll also be saying so at every opportunity, and they may be helped by some in the media; just look at this story from Politico, which says explicitly that even if Obama wins a majority of Americans' votes, he won't have a mandate because not enough of those Americans will be white.
The best thing for Obama to do—which I suspect he would do regardless—is to find whatever creative ways are necessary to work around the House and accomplish all the policy goals he can. While in the past some presidents have been criticized for acting as though they have a mandate they didn't earn (Democrats said this about George W. Bush after the 2000 election), the public only cares about whether your policies are good or bad. No voter is going to say, "I'm glad that now I can get insurance despite being a cancer survivor, but I'm just not sure whether Obama exceeded his mandate by making it so I can do that." Getting re-elected is all the mandate he needs.