A trial started at 1 p.m. Monday challenging the constitutionality of Pennsylvania's voter identification law, one of the strictest in the nation.

11102011-85v_image_1024w Gov. Tom Corbett (AP)

The law, which has never been enforced, was approved last year with the support of no Democrats and all but three Republicans, and signed into law by Gov. Tom Corbett (R). Three groups, the Pennsylvania League of Women Voters, the NAACP and the Homeless Advocacy Project, are challenging the requirement that voters provide photo identification at polling places across the state.While the trial, which is expected to last more than a week, is taking place in Commonwealth Court, it will likely reach the state's supreme court eventually, and the final ruling could have national implications.

Pennsylvania attorney general Kathleen Kane, who last week refused to defend the state's gay marriage ban in federal court, raised the prospect of bowing out of this case as well. But she is going ahead and defending it in court, arguing in a statement Friday the two cases are qualitatively different because the gay marriage law "is wholly unconstitutional. It cannot be fixed."

"The Pennsylvania Voter ID law is, on its face, constitutional," Kane said. "My concern with the Voter ID law has always been its implementation."