TOPEKA, Kan. — Even as Democratic officials fight the effort in court, the Trump administration has run millions of voter registrations through government databases to determine their eligibility in a process that critics worry could end up purging valid voters from the rolls before the November elections.

At least 67 million registrations, primarily from Republican-controlled states, have gone through a beefed-up verification program at the US Department of Homeland Security, and tens of thousands of those have been flagged as potential noncitizens or people who have died. Some states allow only a month for people to prove their eligibility and others suspend it immediately.

The scanning of state voter rolls at the national level is part of a broader effort by Republican President Trump to federalize certain election functions and promote his messaging that elections are marred by noncitizen voting, even though instances of that are rare. Voting and civil rights advocates say the DHS system is error-prone and can mistakenly flag people who are eligible to vote.

“If a voter is wrongly removed, by the time they learn about it and correct it, they may miss their opportunity to vote in that election,” said Freda Levenson, a lawyer with the American Civil Liberties Union of Ohio. The group is challenging an Ohio law requiring monthly checks with the DHS system.

Voters such as 29-year-old Anthony Nel have been caught in the middle.

The native of South Africa, who became a citizen more than a decade ago, was flagged as a potential noncitizen when Texas ran its voter file through the DHS verification system. Nel’s local election office in Denton, north of Dallas, temporarily canceled his registration last fall while he was waiting for a new passport to replace an expired one.

“I’m like, ‘You should know that I’m a citizen, that the passport exists,’” he said in an interview.

Trump has been trying to overhaul US elections, including calling for a federal list of verified voters, and his Department of Justice has pushed states to hand over unredacted voter information for mass checks through the DHS program known as SAVE.

The Justice Department has sued states that refuse, saying the government is trying to ensure that they are complying with federal law and have accurate voter lists. States already take several steps to maintain the accuracy of their voter rolls.

SAVE, short for Systematic Alien Verification for Entitlements, was created under an immigration law mandating that DHS help federal, state, and local agencies prevent government benefits from going to noncitizens. US Citizenship and Immigration Services, an arm of DHS, said more than 1,300 agencies use it.