Washington DC - Democratic leaders have sued President Donald Trump's administration to stop armed federal agents being sent to polling places during November's midterm elections, arguing that even the threat of deployments could intimidate voters.

The party is accusing immigration and law enforcement agencies of asserting authority to send armed officers to voting locations despite longstanding federal restrictions on such deployments.

Chuck Schumer and Hakeem Jeffries, the top Democrats in the Senate and House of Representatives, joined congressional and governors' campaign organizations on the case, filed late Wednesday in federal court in Washington.

The administration has said it has no formal plan to station agents at polling sites, but senior officials have declined to rule out their presence in certain circumstances.

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Homeland Security Secretary Markwayne Mullin said this month that immigration agents could turn up in response to a threat or to serve a warrant on someone they were pursuing.

FBI Director Kash Patel similarly told senators last week that agents would go to polling places "if there's a reason to go there because there's been a violation of law."

And Trump himself was asked in May if he would send National Guard troops or immigration agents to voting locations and would not rule it out.

"Well you know what? I'd do anything necessary to make sure we have honest elections," he told reporters.

Democrats argue that such statements amount to a policy authorizing federal agents at polling locations and risk discouraging voters – particularly naturalized citizens and others who may fear encounters with immigration authorities.

"Voters need not actually be arrested, questioned, or even approached" for the threat to be intimidating, the lawsuit says.

The complaint cites a federal law generally barring armed federal personnel from election sites except when needed to repel armed enemies, as well as a provision of the Voting Rights Act prohibiting voter intimidation.

It asks the court to block armed federal officers from operating around polling places, ballot drop boxes, and other locations where votes are cast, collected, or counted, from early voting through post-election ballot processing.

The Democrats also cite several incidents involving immigration agents around voting sites earlier this year.

The lawsuit follows similar cases filed by civil rights organizations, labor unions, and the city of Denver seeking to restrict federal law-enforcement activity around polling places.