Sean Trende analyzes conflicting state and national polling:
I think the simplest explanation is that the state and national polls really are saying different things, at least for now. In other words, if you are calling for the state polls to be right, you are pretty much necessarily calling for the national polls to be wrong, and vice versa. How do we resolve this? Which will be correct? My best answer is “I don’t know; it is a source of uncertainty in projecting the election.” I suspect one group of polls will converge upon the other in the next week, and we’ll get a better idea.
Nate Cohn agrees that "polls over the last week give more credence to the argument that there isn’t a split between the popular vote and the Electoral College, but instead between the national polls and the state polls":
Yesterday, Georgia, California, Massachusetts, North Dakota, and Montana polls were all consistent with a national popular vote victory for the president. In particular, the California poll is especially important. Perhaps surprisingly, California has been one of Obama's weaker states since the first presidential debate, with several polls showing Obama ahead by 12 to 15 points. In 2008, Obama won California by 24 points and 3 million votes–dropping off 9 to 12 points in California would cut his share of the popular vote by 2 percentage points. Given the diversity of the state, such a large drop-off would be very surprising, even if low Hispanic turnout could plausibly explain some of the decline.
Nate Silver weighs in:
We’re approaching the point where Mr. Romney may need the state polls to be systematically biased against him in order to win the Electoral College. And that certainly could turn out to be the case: if Mr. Romney wins the popular vote by more than about two percentage points, for example, he’ll be very likely to cobble together a winning electoral map, somehow and some way. (And he’ll be a virtual lock if the results are in line with Mr. Romney’s best national polls, like the Gallup survey, which put him four or five points ahead.)
But the historical evidence weighs in slightly more heavily on behalf of the state polls, in my view, when they seem to contradict the national ones. If the state polls are right, than Mr. Obama is not just the favorite in the Electoral College but probably also in the popular vote.
[T]he bottom line is that the final stretch of this campaign is likely to remain something of a mystery, particularly with all the manic spin going on about “who’s winning,” especially on the Right, where confidently predicting a Romney victory or even a landslide appears to be a powerful herd obligation. There’s certainly more than a little incentive for base voters on both sides to take the trouble to vote. In particular, Obama supporters should note that most of those national polls showing Romney ahead among likely voters also show Obama leading among registered voters. GOTV is how you get the former sample to more closely resemble the latter.
Chart from pollster.