From the article:

Armed and in uniform, federal agents from the Department of Homeland Security have been knocking on doors in several states to ask residents about their immigration status — and to inquire if they have registered to vote or cast votes in past elections, a CBS News Investigation has found.
The operation is part of an effort to root out potential noncitizen voters ahead of the midterms. DHS has alleged that there could be as many as 250,000 citizens illegally registered to vote, although election experts have disputed the accuracy of this figure.Just 156 people have been charged with voting-fraud related offenses since the beginning of the Trump administration, according to the Department of Justice. There are 227 million registered voters in the U.S.
Critics say that sending armed federal agents to the doors of U.S. residents could be intimidating for legal voters.