Gary King asked the question “What If the Obama Campaign Didn’t Win Him the Election?” and makes the fair point that Obama’s battleground state vote percentage does not look very different than the other states.
On Friday at the Google Political Innovation Summit, a contingent took the side that because we can’t see the effect of campaigns in analyses like King’s, campaign efforts are generally worthless. I want to make the simple point that Obama’s and Romney’s persuasion efforts can cancel each other out, thus making the effects of campaigns hard to see. Examining turnout, rather than support, shows the impact of campaigns, as both Obama and Romney are attempting to turnout their supporters.  Below I replicate King’s exact graph, but with turnout (VEP) as the variable of interest.
The black, battleground states are closer to the 45-degree line than the non-battleground states, demonstrating the effects of campaigns. Turnout in non-battleground states fell 3.9 points from 2012; the same drop in battleground states was only 1.7 percentage points. This battleground state distinction is exactly what we expect to see in between a high-turnout election (2008) and a less exciting election (2012) if campaigns are able to partially offset the lack of enthusiasm.
 The sustained Voter ID effort by the GOP hurts this hypothesis as these tactics had the potential, if the courts hadn’t struck them down, to cancel out some of Obama’s GOTV efforts. I would note that turnout in Kansas, the only state with a new Voter ID law upheld by the courts, dropped 5.4 percentage points — a good deal higher than the non-battleground state average.
Compelling stuff. It might also be of interest to see what did this, like GOTV versus say advertising, direct mail, and candidate rallies.
Also, scholars tend to argue that down the ballot, campaign effects are stronger. Is there any way to explore campaign effects in Senate races?
You raise a great dichotomy — it’s easier to identify the effects of a presidential campaign because everyone has a ballot line for president, but we’d expect to see larger campaign effects down ballot.
Here’s an example of a paper using the battleground state natural experiment finding a persuasive effect of TV ads in presidential campaigns: http://gbytes.gsood.com/wp-content/uploads/2012/08/AJPS_presads.pdf
As DCCC Targeting Director, I have inside info about the effect of TV and mail on congressional races, but of course those results aren’t sharable (as they are not based on public information).