Political groups and candidates have purchased absentee voting information from the commonwealth on about 300,000 Virginia residents to more deeply target their efforts in a battleground state that could decide the presidential race and which party controls the U.S. Senate.

The State Board of Elections tracks who requests absentee ballots and who casts them and, under state law, provides that information for campaign and political purposes “upon request and for a reasonable fee.” The information includes voters’ names and mail and e-mail addresses, but Social Security numbers and birth dates are redacted.

The data are extremely valuable, especially in battleground states such as Virginia, said Paul Gronke, director of the Early Voting Information Center at Reed College in Oregon.

“Both parties have these long lists with millions of names on them,” said Gronke, whose center tracks information on early, in-person and absentee voting nationally. “It’s a big checklist, and they’re trying to check your name off.”

Since absentee voting began Sept. 19, state election officials have received requests for voter information from the Democratic Party of Virginia, the Republican Party of Virginia, voter data services Catalist and Aristotle, and the campaigns of GOP congressmen Bob Goodlatte and Frank R. Wolf, according to an open-records request filed by The Washington Post.

Nikki Sheridan, spokeswoman for the state election board, said would-be absentee voters have complained about the additional layer of campaigning their ballot request has triggered.

“They seem to be able to correlate it to the fact that the [campaign contact] started to happen after they requested an absentee ballot,” Sheridan said.

Bob Holsworth, a longtime political analyst and former professor at Virginia Commonwealth University, said that although voting is a public act, most voters see their votes as a private matter.

“I’m not sure that most people are aware that the simple request for an absentee ballot and the tracking of that ballot is available on a real-time basis to the parties,” Holsworth said. Voters think of it as “the secret ballot, cast in the sanctity of the voting booth.”

But candidates and campaigns have had access to absentee voter data for decades, Gronke said. “The thing that distinguishes states at this point is not availability but the speed and ease and cost of gaining the information.”

In some states, such as North Carolina, the information is free and downloadable online. In Iowa, the same information is available, but the state charges per voter. Some agencies provide statewide information; elsewhere, it is attainable on a county-by-county basis.

Absentee voting was key to Democrats’ turnout strategy in Virginia four years ago, when they outpaced Republicans on absentee turnout 61 percent to 39 percent. But this year could be different.

Voters have been casting absentee ballots at a faster rate in Republican-leaning parts of the state than in Democratic territory, according to an analysis by the nonpartisan Virginia Public Access Project. The state Republican Party has set up a Web site, www.GOPAbsentee.com, and asks GOP voters to sign up for an absentee ballot “if there is any chance that you might not be able to make it to the polls on November 6, 2012.”

Democratic Party spokesman Brian Coy said he expects a “very aggressive showing” again this year on absentee voting among Democrats. Coy said the absentee voter lists help the party determine who has already voted and whom to target in subsequent election years as likely absentee voters.

“It’s just our way of making sure everybody votes,” Coy said. “Sometimes folks just need a bit of a reminder, and we’re happy to provide that to make sure we maximize the number of folks that participate in this election.”

The result is that campaigns with the resources to collect and process such information are the ones who are able to benefit from it. “The [presidential] campaigns are going to spend that money, but what about a small state legislative campaign, or a challenger to a sitting senator?” Gronke said. “It creates an unlevel playing field.”

Sheridan said that while the practice may upset voters, the agency is following the law.

“It’s unfortunate to me that the people we’re trying to target . . . have a negative experience as a result of participating in the process,” she said. “I can certainly understand their frustration. We implement the code — we don’t write the code.”