By Greg Giroux, CQ Staff

There is no definitive evidence that the pre-Election Day voting allowed in a growing number of states sways the outcomes of electoral contests. But one thing is for sure: the proliferation of “early voting” — including in key battleground states such as Ohio, where voters began to cast ballots on Tuesday — is complicating the strategies and voter turnout efforts employed by candidates and their campaigns.

In a traditional campaign, candidates can build their advertising and get-out-the-vote plans steadily to a crescendo just before that early November Tuesday designated as Election Day. But a candidate in an early voting state has to be cognizant that the messages and voter mobilization tactics implemented as early as September could make the difference between victory or defeat in November.

Granted, of the vast number of voters expected to support Barack Obama , John McCain or third-party candidates in this year’s presidential election — more than 122 million voters participated in the 2004 White House contest — most still will wait to go in person to polling places on the national Election Day of Nov. 4.

Still, 31 states now allow some form of no-excuse, in-person early voting, according to a recent compilation by the National Association of Secretaries of State, an organization of state officials who oversee election laws.

Perhaps one-third of the 2008 electorate will vote before Election Day, counting those who visit “early voting” polling places set up by state authorities and those who use longer-established “absentee ballot” procedures — often limited to those who say they will be out of their home precincts or otherwise unable to make it to a polling station on Election Day — that are available in some form in all 50 states.

In the 2006 midterm elections, the early voting option was used by more than 40 percent of the electorate in Nevada, which is one of the seven states that CQ Politics presently rates as a toss-up in this year’s presidential race between McCain and Obama. Early voters comprised nearly one-fifth of the 2006 electorates in Florida, New Mexico and North Carolina, where Obama and McCain are polling competitively against each other.

Early voting began on Sept. 22 in Georgia, where McCain currently is favored to win, and on Sept. 25 in the key swing state of Iowa, where polls show Obama with an edge four years after the state voted very narrowly in favor of President Bush.

Although no votes will be counted until Nov. 4, both candidates’ campaigns will be even more closely monitoring the turnout in Ohio, where early voting just got under way and where there are highly competitive races for the state’s 20 electoral votes and in four of its U.S. House districts. Bush’s narrow victory in Ohio, by a 2 percentage-point margin in 2004, clinched his re-election.

“We are 35 days from the election, and voting has begun in Ohio,” Kevin DeWine, the deputy chairman of the Ohio Republican Party, said on a conference call with reporters on Tuesday. He added that Ohio Republicans were “absolutely confident in our ability to be competitive during the early voting period.”

The state Republican Party did, however, try unsuccessfully in court to delay the early voting period because voter registration remains open until Oct. 6 — creating a “same-day registration” scenario in which an Ohio resident could register and then go immediately to a polling place to cast an early vote.

The early voting period in Nevada runs from Oct. 18 to Oct. 31. Clark County, which includes Las Vegas and most of the state’s population, has a list of 88 early voting sites, including an outlet mall and athletic club.

There is widespread support among the public for early voting. With Americans working longer hours, voters want more convenient options to cast their ballots than standing in a long line at a polling station on Election Day, which falls on a weekday and isn’t a national holiday.

State election offices like early voting because they can receive and process ballots over a longer period of time. Some of them promote early voting as if they were selling a product to a consumer. For example, the Web site of the Secretary of State’s office in Georgia says: “Is your schedule hectic? Not sure if you’ll be in town on Election Day? Want to avoid Election Day crowds? Then take advantage of one of the numerous voting options offered here in Georgia.”