More: "What we're seeing now is the federal government is inserting itself into the state process for verifying their voter roles in an unprecedented way," said Jessica Marsden, a lawyer for Protect Democracy, a left-leaning advocacy group that is suing the federal agency over the door-knocking campaign. "And they are doing it in a way that seems maximally designed to scare lawful voters from participating."

The home visits ramped up in September, in the thick of the midterm campaign season, and continued over the past weekend. The Justice Department announced Monday that Homeland Security agents in California arrested more than 20 people, all of them green card holders, on allegations they had illegally registered or cast a vote.

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Eric Balliet, a former Homeland Security Investigations agent, said the door-knocking operation is a significant departure from traditional law enforcement activities.

"In my 23 years with HSI, I never engaged in or was aware of any incident or initiative that sent Special Agents to private residences to verify the eligibility of voters," Balliet said, who served in the federal government under multiple administrations.

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For civil rights advocates, the show of force has sparked concerns about the chilling effect it could have on legal voters.

"It is sending the signal to a much broader community, do not show up at the polls. Do not vote," said Dr. Carol Anderson, a professor of African American Studies at Emory University.

"And so if you are somebody who has registered to vote and you're a naturalized citizen, what you see in that community is that " because this is not a regime that makes those discernments " that you're gonna stay away from the voting booth."