The importance of influencing early voting opportunities–particularly in close elections–is a very familiar story in U.S. politics by now, but it’s sometimes easy to forget that this concern has changed the very rhythm of campaigns. RCP’s Alexis Simendinger has a good round-up of the relevant dates for early voting in key battleground states (with Iowa firing the starter’s gun on September 27), and some of the preparations the two presidential campaigns have made. Nearly one of three votes were cast before election day in 2008, and the number is expected to be higher this year. Indeed, over half of the votes will almost certainly be cast early in the battleground states of Colorado, Nevada, North Carolina and Florida, with Ohio not far behind.

The early voting phenomenon has moved the goal posts for presidential candidates and the parties, reshaped the nature of voter mobilization, intensified the focus on battleground states and election laws, and created openings to gauge (and influence) who is winning — and where. It’s also inspired data-driven debates about whether early voting is increasing or decreasing overall turnout.

As Simendinger explains, the campaigns have a strong incentive to encourage certain supporters to vote early:

It may not be commonly understood that tallies of early voting (individual’s names, addresses, party registration) are public and open to examination in many states leading up to Election Day. This information affirms for campaigns and the political parties which individuals cast ballots early, or are in the process of doing so. The campaigns are spending hundreds of millions of dollars on techniques and technologies to reach eligible voters. By registering them to participate, then encouraging them to act early, and then shepherding those supporters to the finish line where they can vote early, the campaigns build cushions under their candidates and gather up valuable data along the way.

Locking up votes early means campaign workers can shift to other targets for support, and deploy money, staff and volunteers more efficiently. The campaigns can encourage voters to avoid Election Day lines and inclement weather, and assist them with voter eligibility issues and ballot questions.

As Simendinger also notes, early voting can blunt the effects of late campaign developments, which in turn means that ads with negative information about candidates will run earlier as well. That may explain why the presidential campaign and some down-ballot contests have gotten nastier earlier than ever, and why the old saw about nothing mattering before Labor Day should be buried along other outdated election lore.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.