The Republican-backed law requires voters to provide certain photo ID before they can cast ballots and is one of the strictest in the nation. Gov. Tom Corbett signed it in March 2012, but courts have blocked its enforcement since before last year's presidential election.
Civil libertarians want it declared unconstitutional.
The trial began Monday in Commonwealth Court and continues Tuesday. It will examine a challenge by voting-rights groups that say the state hasn't done enough to ensure all eligible voters can get the necessary ID. State officials say they've taken all reasonable steps.
Critics say the law is a Republican attempt to discourage voting by Democratic-leaning voters; supporters say it would discourage voter fraud.