Voting rights advocates and some scholars say they have concerns that Mr. Kobach, who maintains that voter fraud is pervasive, may want to build a case for stricter requirements for registering to vote and casting ballots. They also argue that the database comparisons he wants to make will, like the Crosscheck program, vastly overestimate the potential for fraud.

Can the federal government legally demand this information?

Yes and no. The Privacy Act of 1974 bars the federal government from recording citizens’ party affiliations in most cases. But Congress has the power under Article 1 of the Constitution to compel states to produce other data of its choice, said Michael McDonald, the director of the United States Elections Project at the University of Florida.

As for protecting the data, a representative for the panel has said that it will be secured in computers controlled by the commission’s chairman, Mr. Pence. But there’s a potential loophole, Mr. McDonald argued: Collecting the data for a federal report raises questions as to whether it would be publicly available to anyone who filed a Freedom of Information Act request.

Why are so many states balking at providing the data?

A lot of reasons — legal, political and historical.

Many states have laws barring the release of even partial Social Security numbers or birth dates, because of their propensity to be used for identity theft. When Georgia accidentally released full Social Security numbers on CDs containing otherwise public voter data in 2015 — the so-called Peach Breach — the public was appalled. In another instance, Ohio released the Social Security numbers of at least 5.7 million voters — 80 percent of the state’s electorate — because the secretary of state’s office failed to remove them from public data requests. The Ohio secretary of state at that time was J. Kenneth Blackwell, a member of the commission making the current requests.

Many Democratic secretaries of state are resisting because they believe the commission is laying the groundwork for restrictions that will mostly make it harder for traditionally Democratic constituencies — minorities, young people and the poor — to cast ballots. Even the drafters of the Constitution could not agree on nationwide rules for casting votes. For Republicans in particular, the resentment has only increased since the passage of the Voting Rights Act in 1965 and the National Voter Registration Act — the motor-voter law — in 1993.

Isn’t a lot of this information publicly available?

The Help America Vote Act of 2002 requires every state to produce a list of registered voters, commonly known as a voter file, which includes, at a minimum, voters’ names, addresses and a list of elections in which they have participated.

States differ on who may retrieve the voter file. A few, like North Carolina and Ohio, allow anyone to download data from state websites. But New Hampshire and Virginia release data only to political candidates or organizations like political action committees that participate in voter registration or turnout drives.