LUMBERTON, N.C. — It was a triumphant moment for North Carolina Republicans in 2013 when they enacted one of the nation’s most aggressive voter-identification laws.

The measure would combat voter fraud, they argued — though, as federal courts later ruled, it would almost certainly reduce African-American Democratic turnout. At the same time, the law made it easier to obtain mail-in absentee ballots, a form of voting that Republicans used more than Democrats.

But now, with an investigation underway into potential abuse of absentee ballots in a disputed House race, North Carolina’s tangled, partisan history of voting rights and fraud is under a spotlight — and Republicans find themselves on the defensive about whether their reliance on voter identification to combat fraud focused on the wrong source of trouble.

“The history of fraud in North Carolina is mostly in absentee ballots,” said Bill Gilkeson, a former lawyer for the General Assembly who worked on election issues. “That’s where the fraud really happens, and there’s a long history.”