But of those states, polls show that only Pennsylvania and Wisconsin have the real potential to be competitive between Mitt Romney and President Obama; the lion's share of other voter-ID-law states, most in the South, are considered deep "red," very likely to go Republican Nov. 6.
To some extent, neither Wisconsin's nor Pennsylvania's law will be a certainty until further appeals are decided. A judge struck down Wisconsin's law in March, and the state's appeal is pending there. The ACLU and other critics of Pennsylvania's law plan to ask the state Supreme Court to review Wednesday's decision by a Commonwealth Court judge upholding the law.
So far, both presidential campaigns are treating Pennsylvania's 20 electoral votes as up for grabs. Obama currently has a seven-percentage-point edge in Pennsylvania in the Real Clear Politics average of independent state polls. But if, as Democrats contend, the new photo-ID requirements prevent some Democrats from voting, it could come into play if the race tightened.
"It's certainly our fear that it's going to cut down turnout in Philadelphia, and a lot of races are close in the state of Pennsylvania," U.S. Rep. Bob Brady, who also is the city's Democratic Party chairman, told The Inquirer after the Wednesday ruling. "There's a very good chance that there will be some impact."
In Wisconsin, polls have Obama up by five percentage points - but Romney's choice of Wisconsin U.S. Rep. Paul Ryan as running mate has Republicans vowing to deliver that state's 10 electoral votes.
Academic studies that have sought to evaluate effects of the stricter ballot-access laws, such as those that require photo ID, estimate turnout decreasing by an average of about 2 percent of registered voters in a state.
While foes of Pennsylvania's law have focused on whether Democratic-leaning groups such as students and minorities are most likely to be affected, "Republicans and Democrats alike could lose their right to vote this election if the law isn't struck down," said Yuri Beckelman, spokesman for the AFL-CIO. He warned that "confusion" about the law might discourage some from trying to vote.
Just how many votes are at issue is hard to guess. State officials produced a list of more than 750,000 registered voters who lacked the two main forms of photo ID acceptable under the law - driver's licenses or nondriver Department of Transportation photo ID. That exceeds the roughly 600,000 votes by which Obama carried Pennsylvania in 2008.
But the accuracy of the state list has since been questioned, and, as state officials point out, those without photo IDs can still try to obtain one by Nov. 6 - or cast provisional ballots if they produce ID within six days.
"It's a complicated calculation," said Tom Lindenfeld, a national Democratic consultant who specializes in turnout and has extensive experience in Pennsylvania races. "Clearly, there's a theoretical potential for it to tip the outcome of the election, but what is the likelihood of that happening? That's a very hard question to answer."
Yet no one in politics can forget the 2000 race, in which Florida went to George W. Bush over Al Gore by 537 votes after the U.S. Supreme Court halted a recount.
That year, the Electoral College vote was so close, 271-266, that a shift in nearly any state would have changed history. The count in 2004 was nearly as close: 286 for Bush, 251 for Democrat John Kerry.
"Even if the effects of voter ID are small, they can still matter," Richard L. Hasen, an expert in election law at the University of California, Irvine, wrote in a March column in the Fort Worth Star-Telegram. "Perhaps voter-ID laws deter about 1 percent of voters from voting (and more who forget their IDs and don't return to vote again - a real concern for Democrats about low-motivation voters).
"One percent may not seem like much," Hasen wrote, "but we have had enough razor-thin elections in recent years that some . . . could well turn on such a small effect."
Both parties are closely monitoring the legal disputes. As Lindenfeld put it, "We've got beaucoup people mobilizing around this."
Contact Thomas Fitzgerald
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