Voters in red and blue states could have very different experiences in 2016.

Millions more Californians could head to the polls for the first time next year, thanks to a law passed by the Democratic legislature and signed Oct. 10 by Democratic Gov. Jerry Brown, that will automatically register eligible citizens when they renew or obtain a driver’s license.

In Illinois, a new provision allows voters to register electronically when they visit various state agencies. And in Delaware, some residents with criminal records will regain the right to vote in the presidential election due to a constitutional amendment passed by the state’s Democratic-controlled legislature and signed by its Democratic governor.

In Republican-controlled states, the story is different. North Carolina has instituted a new voter ID requirement. North Dakota has narrowed the forms of identification voters can present to gain access to the polls. And Ohio’s GOP-controlled legislature has instituted a new set of voting restrictions since the 2012 election, including shorter early voting hours.

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The ballot has been a political battleground since the dawn of the republic. But the voting-rights arms race that ramped up during the Obama Administration will be on full display in November 2016, when 15 states will have new restrictions in place for the first time during a presidential election. “We have states that are going backwards,” says Katherine Culliton-González, director of voter protection at the nonpartisan Advancement Project, “and we have states that are advancing voting rights.”

Since 2010, 21 state legislatures have enacted new laws to curtail ballot access, while 23 others plus the District of Columbia have passed laws to expand it over the past three years. The seesaw struggle reflects efforts by partisan state legislatures to reshape the makeup of the electorate. In most cases, blue states have pushed to expand voting rights, while many of the new restrictions have come in red states.

Recent efforts to restrict voting rights—there were 180 different bills introduced across 41 states in 2011 and 2012 alone—have a disproportionate impact on demographics like blacks, Latinos and the poor, voting-rights experts say.

Read More: Will Democrats Keep Their Early Voting Advantage in 2016?

Conservatives say initiatives like tougher voter ID laws are designed to stamp out fraud and safeguard the integrity of elections. But they also tend to burden the same coalitions that helped lift Obama into the White House, and which seem poised to give Democrats a lasting demographic advantage. Some of the most aggressive changes came in states previously subject to Department of Justice scrutiny, a standard rolled back by the 2013 U.S. Supreme Court case Shelby v. Holder, which struck down part of the longstanding Voting Rights Act.

“We saw a lot of politicians trying to manipulate the rules of the game, such that some people could participate and some can’t,” says Myrna Perez, deputy director of the Democracy Program of the nonpartisan Brennan Center for Justice at New York University School of Law.

A battle that has played out in state capitals around the U.S. is now shifting to the presidential campaign trail. The two leading Democratic candidates, former Secretary of State Hillary Clinton and Vermont Sen. Bernie Sanders, have called for each state to automatically register eligible voters once they turn 18. Clinton sharpened that message during an Oct. 17 stop in Alabama, which recently announced that several government offices that issue driver’s licenses would be closing. (Alabama’s Republican governor says the closures are nothing more than a cost-cutting measure, but advocates say it’ll keep blacks from the polls.)

Invoking the state’s Jim Crow legacy, Clinton blasted the voting rights positions of Republican rivals like Ohio Gov. John Kasich, who has signed a series of laws that critics say were designed to suppress voting, and former Florida Gov. Jeb Bush, who recently came out against reauthorizing a provision of the Voting Rights Act that was struck down in Shelby, arguing high levels of voter participation among blacks in states like Georgia prove it is no longer needed.

Read More: Jeb Bush Explains Why He Opposes Voting Rights Act Provision

“We have to defend the most fundamental right in our democracy, the right to vote,” Clinton said. “No one in this state, no one, should ever forget the history that enabled generations of people left out and left behind to finally be able to vote.”

Demographics could be destiny for Clinton, who is attempting to reassemble Obama’s winning coalition. A lawyer for her campaign is part of a group that has challenged voting statutes in Wisconsin and Ohio, arguing they burden young, black and brown voters. Some observers say those lawsuits double as a political ploy to juxtapose the Democrats’ efforts to expand voting access with the GOP’s attempts to restrict it.

In key 2016 swing states like Ohio and North Carolina, new barriers to the ballot box could produce low turnout that benefits Republicans. “It’s going to be challenging already to get people out to vote,” says Marcia Johnson-Blanco of the Lawyers Committee for Civil Rights, “and when you layer that with efforts to keep people from voting, what you end up with is limited opportunity for participation in our democracy.”

Supporters of policies like automatic voter registration frame it as part of a movement to modernize voting that transcends the partisan divide. Small technological advancements, like online registration, have been adopted in both blue and red states. In key swing states like Florida and Virginia, online registration laws passed Republican-controlled legislatures and were signed by GOP governors.

Read More: Democrats in Selma Gear Up for Long Fight on Voting Rights

Automatic registration is projected to significantly expand the electorate in California, where state officials estimate 6.6 million eligible residents are not registered to vote. The change comes on the heels of a similar new law in Oregon, which was signed in March by Democratic Gov. Kate Brown. California could become a catalyst for the states that have introduced similar bills.

“It’s quite significant,” says Richard Hasen, a law professor at the University of California, Irvine and author of The Voting Wars. “If California is successful, then we may well see states follow suit.”

But the issue still splits on partisan lines. Though New Jersey’s Democratic-controlled legislature recently passed an automatic-registration bill similar to those in Oregon and California, Republican Gov. Chris Christie is not expected to sign it. “I don’t think that people should be automatically registered to vote,” Christie said June 25. “Is it really too much to ask to ask somebody to fill out a form to execute on their right to vote?” (A Christie spokesman declined to comment on pending legislation, but noted New Jersey has a strong voter registration rate.)

Perhaps sensing the charged politics at play, supporters describe automatic voter registration as a matter of sound policy. It is designed to make the process more inclusive and efficient, says Perez of the Brennan Center. Automatic registration shifts the responsibility for registration from citizens to the government, and is designed to streamline the voting process by electronically linking the driver’s license database with voting databases. The change is intended to clean up the rolls by reducing human error, such as names improperly entered because one election official could not decipher another’s handwriting.

“We as a country don’t do enough to get eligible voters on the rolls,” Perez says. “We think this is the modern way of doing it.”

Read Next: The Voting Rights Act at 50: How the Law Came to Be

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Write to Alex Altman at alex_altman@timemagazine.com.