Tens of thousands of voters were registered to cast ballots in both Virginia and Maryland during the 2012 presidential election — and more than 150 appear to have voted twice, an advocacy group claims.

Seventeen of those alleged instances were in Fairfax County, where election officials found the evidence so compelling that they have turned the information over to law enforcement.

The situation sparked a strong reaction among some political leaders in Virginia, coming in the midst of a heated national debate over whether voter fraud is rampant or mere rhetoric.

Many Republican-dominated state legislatures, including Virginia’s, have passed strict new voter identification laws in recent years that their supporters say are intended to reduce voter fraud. But Democrats claim that actual fraud is rare, and they say the real purpose of ID laws is to restrict ballot access for such groups of voters as the elderly, college students and immigrants who are less likely to hold a government ID — and more likely to vote for Democrats. But an ID wouldn’t have made a difference in this case.

“These are alarming allegations that, if true, could undermine the integrity of our electoral processes,” read one statement issued by Speaker William J. Howell (R-Stafford) and other Republican leaders of the Virginia House of Delegates. Pat Mullins, chairman of the Republican Party of Virginia, called for a full investigation and prosecution of offenders “to the fullest extent of the law” if it is determined that any voters cast multiple ballots in the same election.

Some Democrats were more skeptical, saying the claims need to be checked carefully. “The GOP has been on a witch hunt for voter fraud for 15 years to make it harder for people to exercise their constitutional rights,” said Del. Scott A. Surovell (D-Fairfax). “They want to pretend their true motive is electoral purity, when the reality is they’re just trying to prevent Democratic voters from getting near a voting machine.”

Jeremy Mayer, an associate professor at George Mason University, said that one of the challenges of policing voter fraud is the absence of a national registry of voters. It’s a federalist, states’-rights tradition that leaves a confusing mishmash of laws, rules and records, he said. When people move to another state, most don’t cancel their previous registration, he said, and a mix of local and state officials take on the task of slogging through registrations to verify that voters are still living at the same address.

“That’s a very time-consuming, budget-heavy thing to do,” Mayer said.

Reagan George, whose advocacy group Virginia Voters Alliance discovered the apparent fraud, said he decided with the head of a similar group, Election Integrity Maryland, to compare registration lists for the 2012 presidential election. The places with the most duplicate registrations were the suburban counties surrounding the District, and 164 voters appeared to have voted in both states that day.

Because of concerns about outdated and inaccurate voter registration lists, seven states — including Maryland and Virginia — formed a consortium after the 2008 election with the help of Pew Charitable Trusts to analyze data across state lines and update voter rolls more efficiently.

The Electronic Registration Information Center, which now includes 12 states and the District of Columbia, helps states maintain more accurate lists of voters by using car registrations and other data. The consortium identified more than 60,000 people who had died but remained on active registration lists, said David Becker, director of Pew’s election initiatives.

Another national initiative, the Interstate Crosscheck, uploads data after an election, so it’s particularly good at identifying voters who cast ballots in more than one state. More than half the states now participate in the Interstate Crosscheck, which started in Kansas.

Edgardo Cortés, commissioner of the Virginia State Board of Elections, did not respond directly to questions about this case, but in a written statement, he said that the state has a “robust” program for sharing data with other states and follows a policy of referring all potential violations to investigators.

Nikki Charlson, deputy administrator of the Maryland State Board of Elections, said Virginia is taking the lead on the investigation. She noted that the numbers likely reflect both clerical errors and many voters being classified as inactive, a process governed by federal law that requires four years before they are stricken from the rolls. Maryland and Virginia participate in the Pew effort to cross-check voter registrations state to state.

George said he has heard from county officials eager to check into their own cases.

Brian Schoeneman, secretary of the Fairfax County Electoral Board, looked into the 17 cases flagged locally, comparing them with data provided by Montgomery County officials. A couple cases involved absentee ballots, but most of the people appeared to have physically crossed state lines to vote twice, he said. “If these are the same folks, they knew what they were doing. It’s a Class 6 felony.”

One of the cases involves someone who appears to have voted in both Maryland and Virginia for the past decade, George said.

Members of Fairfax’s Electoral Board asked the Fairfax County commonwealth’s attorney, the state attorney general and the U.S. Department of Justice to investigate the 17 people for possible voter fraud.

“Seventeen potential voters out of 700,000 registered voters in Fairfax — it’s a drop in the bucket,” Schoeneman said. “But one is too many.”