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What's Behind the State-National Poll Divergence?

What's Behind the State-National Poll Divergence?

By Sean Trende - October 31, 2012


The Hurricane Sandy-related lull in tracking polls gives us a good opportunity to look at one of the more interesting aspects of polling to develop this month: the divergence between national and state polls. Put simply, the national surveys point to a Romney win, while the state polls collectively point to an Obama win. Both can’t be correct.

The RCP Average currently has Mitt Romney up by 0.8 points nationally. He has held this lead fairly consistently ever since the first presidential debate.

Given what we know about how individual states typically lean with respect to the popular vote, a Republican enjoying a one-point lead nationally should expect a three-to-four-point lead in Florida, a two-to-three-point lead in Ohio, and a tie in Iowa. Instead we see Romney ahead by roughly one point in Florida, and down by two in Ohio and Iowa.

Of course, every cycle sees states shift their partisan leans, sometimes radically. Nevada has swung heavily toward Democrats in a relatively short time, while West Virginia has bolted for Republicans. So it wouldn’t be shocking to see the partisan lean in Florida, Ohio and Iowa shift leftward. Indeed, it may be that Obama’s ad war, ground game, and policy decisions over the past four years (i.e., the auto bailout) have “frozen” these states in place vis-à-vis the rest of the country.

The problem is that shifts in partisan lean are zero-sum games: If one state moves rightward relative to the popular vote, some other state or states has/have to move leftward. (If you doubt me, try constructing a scenario in which all states are to the left of the national vote.) To account for what we see, some have hypothesized that Romney is simply over-performing in the blue states and blowing the roof off in the Southern red states, both of which are untouched by Obama’s ground game.

But this theory immediately runs smack into the face of some inconvenient facts. For starters, Florida, North Carolina, and Virginia collectively represent about one-third of the population of the South, so if the “state poll view” of the races is correct, then it becomes difficult for Romney to run up huge margins in the South. He would have to really be doing well in places like Texas, Mississippi and Georgia.

But this doesn’t square with the reality of the Deep South with Obama atop the ticket. Georgia and South Carolina were single-digit races in 2008, and the latest poll has Romney up only eight in the Peach State -- just a three-point improvement over McCain’s 2008 showing. In places like Mississippi and Alabama, the GOP likely maxed out its potential vote share against Obama in 2008; McCain won 88 percent of Mississippi whites while Obama won 98 percent of the state’s blacks. There simply isn’t much room for the GOP to grow under those circumstances. Texas? McCain won 73 percent of the white vote. So unless the GOP makes big gains among Latinos or turns Austin red, Texas is unlikely to be a vote sink for Romney. There are only so many votes the remaining states, like Tennessee and Kentucky, can provide.

But rather than provide anecdotal evidence, we can reverse-engineer a national poll from the state polls, and compare that to actual national polling. Since the national vote is a collection of state votes, polls of all states should collectively approximate the national vote (since errors should be randomly distributed, they should cancel out). This is done by a simple weighted average. First, I took the states with RCP Averages or, if available, polling from October. This provides data for 31 states. By keeping the poll data post-Oct. 4, our data set is confined to a fairly stable period in the race.

For the 19 remaining states, I assumed Romney would do as well as George W. Bush, who won nationally in 2004 by a little over two points. This is probably a fairly generous estimate for how Romney will do in these states, since I find it highly unlikely that he will match Bush’s 20-point win in Mississippi, due to higher minority turnout.

There were two exceptions to this approach: I downgraded Romney a few points from Bush's showing in Texas, and upgraded Obama a few points from Kerry's Hawaii performance, owing to the “home-state advantages” the former now lacks and the latter now holds.

I took these percentages and multiplied them by the actual number of votes cast in each state in 2008. This produced an expected vote total for Obama and Romney in each state under current polling (or, if not polls were available, under Bush’s 2004 vote shares).

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Sean Trende is senior elections analyst for RealClearPolitics. He is a co-author of the 2014 Almanac of American Politics and author of The Lost Majority. He can be reached at strende@realclearpolitics.com. Follow him on Twitter @SeanTrende.

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