President Donald Trump has criticised the growing number of US states refusing to pass on voters' details to his commission on electoral fraud.
"What are they trying to hide?", Mr Trump tweeted.
At least 20 states have said that they will not or only partly comply with the request, citing privacy concerns.
Democrats fear that the commission may be used to justify tightening voting procedures - changes which could make certain groups less likely to vote.
The groups most affected by so-called voter suppression tend to vote Democrat.
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But it is not just Democrats who are opposed to the collection of such data by the federal government.
Mississippi's Secretary of State, Delbert Hosemann, a Republican, said in an official statement that his reply to the commission would be "they can go jump in the Gulf of Mexico and Mississippi is a great state to launch from".
Mr Trump set up the commission to investigate claims - unsubstantiated, but which he repeats - that millions of fraudulent votes cost him the popular vote in the 2016 presidential election.
He secured more votes in the all-important electoral college than his Democratic rival, Hillary Clinton, thus winning the presidency.
Mr Trump established the Presidential Advisory commission on Election Integrity in May, despite evidence that voter fraud is not a widespread problem in the US.
The aim is to "increase the American people's confidence in the integrity of our election systems".
Kentucky's Secretary of State, Alison Lundergan Grimes, said she would not be releasing "sensitive personal data to the federal government".
"Kentucky will not aid a commission that is at best a waste of taxpayer money and at worst an attempt to legitimise voter suppression efforts across the country," she said in a statement.
The panel, described by Mr Trump as "very distinguished," is chaired by Vice-President Mike Pence.
On Wednesday its vice-chair, Kansas Secretary of State Kris Kobach, sent a letter to the 50 US states and the District of Columbia requesting details from voter rolls including: names, addresses, dates of birth, political affiliation, last four digits of social security number, voting history since 2006, criminal convictions and military status.
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The information would be used "to fully analyse vulnerabilities and issues related to voter registration and voting," the letter said.
Many other states, including California, Connecticut, Minnesota and Alabama have said they will not send the information, or will only send information that is already publically available.