The House passed a controversial bill on Thursday that would require Americans to show proof of citizenship in person to register to vote in federal elections, which critics say could disenfranchise millions of eligible voters.

Four Democrats joined all Republicans to pass the measure in a 220-208 vote.

The bill, titled the Safeguard American Voter Eligibility Act, or the SAVE Act, would require applicants to present documentary proof of U.S. citizenship such as a valid passport, a birth certificate or other forms of identification that show a person is a U.S. citizen. It’s aimed at preventing noncitizens from voting — something which is already illegal and rare.

Republican Rep. Chip Roy of Texas, who introduced the bill, said Thursday “there’s nothing controversial about saying that you should be able to ensure that only citizens vote.”

A 2023 study from the Brennan Center for Justice found that more than 21 million American citizens did not have easy access to documents that would prove their citizenship. Another 3.8 million did not have the documents at all, often because they had been lost, destroyed or stolen. The bill’s requirement to show proof of citizenship in person would affect millions more voters who do have the documentation but register by mail or online, the Brennan Center said.