Years of hard work, billions of dollars' worth of spending, and an incalculable amount of rhetorical grandstanding will face the ultimate political test tomorrow, when millions of Americans head to the polls to select the future president of the United States. The race, however, is far from over: In the final hours of the 2012 campaign, both sides continue to furiously pound the pavement in battleground states across the country, aggressively pursuing the sliver of unpersuaded, unenthusiastic, or otherwise available voters who could decide the outcome of the presidency.

This year, as every election, both campaigns will deploy thousands of volunteers on Election Day and the 72 hours prior to get in touch with voters and try to convince them to turn out on November 6. But for both President Obama's campaign and the Republican National Committee, which is helming Mitt Romney's ground game, the get out the vote effort will have started far before then.

The impact of the early vote

In 2008, the Obama campaign dominated early voting to such an extent that in some critical battleground states - including Florida and North Carolina - he was able to clinch a victory despite Republican John McCain outpolling him among those who voted on Election Day. This time around, Republicans are determined not to let that happen again: The RNC is touting an aggressive early voting program that they say parallels that of the famously well-organized Obama campaign. As a result, both sides have effectively been working their get out the vote efforts for a month.

"People used to call it the final 72 hours. Now that early voting is so prevalent, it's become a much longer get-out-the-vote effort," said Kirsten Kukowski, a spokesperson for the RNC. "In some states, it's been going on for several weeks."

Adds an Obama campaign official: "Every day has been get-out-the-vote for us in those states that allow in-person early voting. We're basically under the operating assumption that every day that we can undertake as robust a get-out-the-vote as possible, we're going to do that."

The benefit of strong early voting numbers is that it essentially extends the timeline of a campaign's get-out-the-vote effort, and allows campaigns to cross known early voters off their list of targets and zero in on those they have yet to reach. That means that, if they've done a good job, by Election Day they'll have a smaller and more focused group of voters to contact.

"Early voting is a boon to campaigns not because it represents in and of itself a way to increase turnout, but because it allows campaigns to allocate their resources more efficiently," said Donald Green, a political science professor at Columbia University who specializes in voting behavior. "If a substantial number of people vote early, those people need not be targeted with resources thereafter. The potential list of targets will be shorter."

Not only that, but targeting certain groups for pre-Election Day voting enables campaigns to concentrate their day-of efforts geographically as well. This may be particularly true for Democrats, many of whom are geographically clustered in urban areas, according to Green.