Defendants in a case against one of the nation's strictest voter ID laws in Pennsylvania made a major concession to plaintiffs this week, just days ahead of the start of the trial over the measure.

In a stipulation agreement signed earlier this month, state officials conceded that they had no evidence of prior in-person voter fraud, or even any reason to believe that such crimes would occur with more frequency if a voter ID law wasn't in effect.

"There have been no investigations or prosecutions of in-person voter fraud in Pennsylvania; and the parties do not have direct personal knowledge of any such investigations or prosecutions in other states, the statement reads.

According to the agreement, the state will not offer any evidence in this action that in-person voter fraud has in fact occurred in Pennsylvania and elsewhere, nor will it "offer argument or evidence that in-person voter fraud is likely to occur in November 2012 in the absence of the Photo ID law.

The possibility of voter fraud has frequently served as the ideological underpinning for voter ID measures, whose supporters claim that the integrity of elections can't be preserved without requiring would-be voters to verify their identity at polling places. Reports on actual incidents appear to counter this contention, however, as figures suggest voter fraud is a highly infrequent occurrence.

Opponents of voter ID laws argue that such legislation is an effort to establish obstacles for potential voters, particularly college students, minorities and the elderly, who tend to vote Democratic. A recent report from the Brennan Center for Justice found that a variety of factors could seriously hamper the ability of a half-million Americans in 10 states that have passed voter ID laws to obtain the required documents they would need to cast votes in November.

Pennsylvania GOP House Majority Leader Mike Turzai fueled the concerns of anti-voter ID activists earlier this year when he claimed that the recently enacted measure would "allow Gov. [Mitt] Romney to win the state of Pennsylvania."

Just weeks after those comments, Pennsylvania officials released a study finding that more than 758,000 registered voters in the state -- many of them in its urban center of Philadelphia -- lacked driver's licenses. While the law allows for a variety of other forms of identification to be used at polling places, the figure suggested that a large number of Pennsylvanians still didn't meet the criteria needed to cast ballots in the fall.

A lawsuit filed against the state's voter ID law by the ACLU and NAACP on behalf of lead plantiff Viviette Applewhite, a 93-year-old woman who claims she will be disenfranchised by the legislation because she won't be able to get valid documentation before the election, is set to go to trial on Wednesday. On Monday, the U.S. Department of Justice also announced that it was investigating whether the law discriminates against minorities.