At Larry J. Sabato’s Crystal Ball, Alan I Abramowitz mines some relevant data to ask and answer a question of consequence: “Do Presidential Campaigns Matter? Evidence From the 2008 Election.” His method:

In order to answer the question of whether presidential campaigns matter, I analyzed evidence from the 2008 election. In 2008, just as in 2012, the presidential campaigns focused their efforts overwhelmingly on voters in a relatively small number of swing states. According to spending data compiled by CNN, 15 states accounted for almost 90% of total spending on television advertising by the Obama and McCain campaigns. These same 15 states were also heavily targeted for grassroots voter registration and get-out-the-vote drives by the campaigns. According to data compiled by Nate Silver of FiveThirtyEight.com, as of early August, more than 80% of Obama field offices and more than 90% of McCain field offices were located in these states.

For openers, Abramowitz notes,

…Altogether, the Obama campaign and its allies spent about $258 million on television ads in these 15 states, compared with about $164 million by the McCain campaign and its allies, a better than three-to-two advantage.
Perhaps reflecting its greater emphasis on grassroots campaigning and ability to capitalize on the enthusiasm of its supporters, the Obama campaign had an even bigger advantage when it came to field organization in the battleground states. As of early August, according to Nate Silver, the Obama campaign had opened 281 field offices in these 15 states, compared with only 94 for the McCain campaign, almost a three-to-one advantage.

Noting that “the size of these advantages varied considerably from state to state,” Abramowitz performs a regression analysis and adds ‘…According to the results of the regression analysis, every 1% increase in Obama’s share of advertising spending in a state increased his share of the vote by about .08 percentage points, and every 1% increase in Obama’s share of field offices in a state increased his share of the vote by .06 percentage points.” Further,

Based on these results, it is likely that both advertising spending and field organization affected the election results in these 15 swing states. And the size of these effects appears to be large enough to have tipped two states that would otherwise have voted for John McCain into the Obama column: Indiana and North Carolina. We can estimate that the combined effects of Obama’s advantages in advertising spending and field organization increased his share of the vote by almost six points in Indiana and by almost five points in North Carolina. And a massive 27-7 advantage in field offices almost put Obama over the top in Missouri: adding almost two points to Obama’s share of the vote and left him just short of victory.

It’s an impressive case for the power of campaigns, as Abramowitz concludes:

It is far from certain whether we can expect the same sorts of campaign effects in 2012. It is probably unusual for one candidate to enjoy substantial advantages in both advertising spending and field organization in a presidential election as Obama did in 2008. Neither the Obama campaign nor the Romney campaign is likely to enjoy a decisive advantage in ad spending this year. And an election with an incumbent running for a second term may be different from one without an incumbent. Nevertheless, the findings presented here suggest that under some conditions, presidential campaigns can affect election outcomes in swing states.
These results also suggest that the impact of field organization can be just as great as that of spending on TV ads. The Obama campaign enjoyed an even larger advantage in field organization than in advertising dollars in 2008, and the findings presented here indicate that this advantage played a major role in Obama’s victories in Indiana and North Carolina and almost turned Missouri blue for the first time since 1996. Given the relative costs of field offices and TV ads, investing in field organization in the battleground states may be a more efficient use of campaign resources than spending on television advertising.

Take note Democrats. Contribute what you can, and, perhaps more importantly — get involved in GOTV.