In a twist, same-day voter registration in a few swing states could help Republicans — a  party that has long advocated against the policy — in the 2024 election.

Last year, North Carolina Republicans passed a measure that would place increased restrictions on voters seeking to register to vote and cast a ballot on the same day in the state. They even overrode the governor’s veto of their new rule to do so.

Although the law was put on hold by a federal judge in North Carolina in early 2024, the law is one example of Republicans having attempted to either restrict or entirely do away with same-day voter registration.

The catch is that restricting same-day voter registration might hurt the GOP more than it helps this year. Keeping with North Carolina as an example, Republicans accounted for about 30 percent of registered voters in the state in 2020. Yet Republicans accounted for about 37 percent of voters who registered to vote and cast a ballot on the same day in the state in 2020.

Democrats, for contrast, accounted for about 33 percent of voters in the state in 2020 and just less than 34 percent of voters who took advantage of same-day registration to cast a ballot that year.

This means that, though Republicans have historically advocated against policies like same-day voter registration, they actually disproportionately benefited from the program in 2020 in North Carolina and might stand to do so again in 2024.

A similar situation played out in New Mexico in 2022, in one of the first elections in the state after the rollout of same-day voter registration. In the 2022 elections, 5,225 Republicans took advantage of same-day voter registration in New Mexico versus 4,521 Democrats, according to the secretary of state’s office.

The associate director at the University of Massachusetts Lowell Center for Public Opinion, John Culverius, explains that though voters who vote frequently — high-propensity voters — have historically skewed toward the Republican Party, in recent years the most reliable voters have actually skewed toward supporting Democrats.

This is part of the reason why Democrats have dominated most special elections in recent years and have overperformed expectations in every special election in the past two years and in the 2022 midterms.

Mr. Culverius cited Virginia as another state where rules making voting easier likely helped Republicans. In the 2019 Virginia elections, Democrats won the trifecta in state government and passed a slate of laws making it easier to vote — laws that likely went on to help Republicans in the 2021 elections that saw the GOP take control of the governorship and the state house.

“A lot of states that will decide the election have these same day voter registration processes,” Mr. Culverius says, adding that “electorates don’t always look like an electorate that has voted in the past or how they have self-reported that they’ve voted in the past.”

This year, key swing states like Michigan, Wisconsin, and Nevada all have same-day voter registration on Election Day. North Carolina also has same-day voter registration but only during the early-voting period.

While Republicans might stand to benefit electorally from policies like same-day voter registration — and they may even take advantage of it more frequently than other groups — polling suggests that most of the party is suspicious of it.

A survey by Politico and Morning Consult found that 35 percent of Republicans support same-day voter registration while 51 percent oppose the policy. Among Democrats, 78 percent support it and 10 percent oppose the policy.