© The Associated Press A voter marks his ballot in Elk Grove, California. Most states offer people the opportunity to register to vote when they apply for a driver’s license, but California and other states with automatic registration just go ahead and do it for them.

Editor's note: This story has an updated number of states with online voter registration.

At least 35 million Americans who are eligible to vote are shut out of the democratic process because they aren’t registered. Can tweaking a 21-year-old law add millions of them to the voter rolls?

That’s the idea behind “automatic registration,” which five states have adopted and two dozen others have considered in the last two years.

Jonathan Brater of the Brennan Center for Justice, a public policy institute at the New York University School of Law, said voter ID laws may get more attention, but getting more Americans registered to vote could have the biggest impact on elections.

“Registration is one of the biggest barriers to voting,” Brater said. “Every election, thousands of people go to the polls and can’t vote because of complications with registration.”

Under the federal Motor Voter Act, which went into effect in 1995, states must offer to register any eligible citizen who seeks a new driver’s license or public assistance. But in many states, the law hasn’t fulfilled its potential, in part because the process often trips up would-be applicants and many state workers don’t consider it a high priority.

States that have switched to automatic registration don’t just offer people the opportunity to register — they go ahead and do it for them whenever an eligible voter applies for a license, without requiring the potential voter or a DMV clerk to take any additional action. People can opt out of registration, but they don’t have to opt in.

“Motor voter was always the homework we didn’t want to do,” said state Rep. Chris Pearson of the Vermont Progressive Party, who sponsored the legislation there. “Motor voter was never the focus of people at the DMV. This in a way turns it on its head. This isn’t, ‘Oh, I need to remember to ask them if they want to register to vote.’ This is about updating our system to make it automated.”

In 2015, Oregon became the first state to make the change. Already, automatic registration has added more than 200,000 people to the voter rolls in that state, an increase of nearly 10 percent.

California, where about 7 million people are eligible to vote but not registered, also approved automatic registration last year. The state is still setting up its program, but the Public Policy Institute of California recently estimated that it could yield more than 2 million new voters, a potential increase of more than 10 percent. The think tank also predicts that automatic voter registration would diversify the electorate, making it more closely resemble the population of the state.

Lawmakers in Vermont and West Virginia approved automatic registration this year, and Connecticut adopted it administratively.

Automatic registration is one of several strategies states are employing to enroll new voters. Thirty-three states now allow people to register online, and 13 have enacted same-day voter registration, which allows people to register on Election Day.

In states that don’t allow same-day registration, supporters of automatic registration say it will eliminate a common Election Day pitfall: people who arrive at the polls thinking they are registered, only to discover that they aren’t, or that they forgot to change their registration after a move.

But opponents, including Republican Gov. Chris Christie of New Jersey, who in August vetoed an automatic voter registration bill in his state, contend that connecting registration to driver’s licenses, which are available to people who are not U.S. citizens, opens the process to fraud. Inevitably, Christie argues, some people ineligible to vote will be registered by accident. Others say people who haven’t bothered to register are unlikely to show up at the polls.