THE DAY after last fall’s presidential election, Kris Kobach got to work. In an email plotting action items for the new Trump administration, Mr. Kobach, the Republican secretary of state in Kansas and a champion of voter suppression campaigns there and nationally, said he had “already started” drafting a key legislative change that would enable states to impose rules complicating registration for millions of new voters — exactly the sort of rules he had advanced in Kansas, with mixed success.

Writing to a Trump transition official, Mr. Kobach said he was preparing an amendment to the National Voter Registration Act to allow states to demand documentary proof of citizenship for new registrants. Despite years of litigation and adverse rulings from courts, that same requirement in Kansas, in effect since 2013, had blocked more than 30,000 people at least temporarily from registering and, in thousands of cases, from voting, according to the Brennan Center for Justice, which studies voting issues and has contested Mr. Kobach’s moves in Kansas.

Nearly all of those blocked in Kansas were eligible U.S. citizens who simply lacked ready access to passports, birth certificates and other documents, as at least 5 percent of Americans do. Disproportionately, those lacking such documents are minorities and younger voters — groups that tend to back Democrats.

Mr. Kobach now leads a presidential commission on election integrity, established by President Trump after his groundless assertion that 3 million to 5 million people voted illegally last November. The commission, stacked with Kobach clones who have made voter suppression into a political cottage industry, could undertake various forms of mischief intended to impede voting. Few would be as effective, or as damaging to electoral participation, as fiddling with registration by changing the NVRA, known as the “motor voter” law.

Enacted in 1993, the “motor voter” measure makes registration as foolproof and easy as possible by allowing people to sign up to vote when they apply for or renew driver’s licenses. The law requires registrants to sign a form attesting to their U.S. citizenship, under penalty of perjury. But in Kansas, and before that in Arizona, voter-obstructing Republicans demanded additional documentation.

The requirement was a solution to a non-problem. In Kansas, a federal court found that in the 18 years before 2013, when the state rule went into effect, just 14 noncitizens attempted to register, and only three actually cast votes in federal elections.

But because many native-born and naturalized citizens lack documents such as passports, the law tripped up huge numbers of Kansans trying to register. In motor vehicle offices alone, where about 40 percent of Kansans sign up to vote, some 18,000 otherwise qualified applicants were blocked from registering, at least temporarily. At least 12,000 others who attempted to register elsewhere had similar problems; many of them were unable to vote in last year’s primaries and general election.

That’s why fears about Mr. Kobach’s intentions now are justified. If his commission endorses the Kansas model, or even recommends requiring documentary proof of citizenship as a condition of voter registration, millions of Americans will face disenfranchisement, and democracy itself will be at risk.