Washington (CNN) Former Vice President Joe Biden's campaign has submitted a formal request for Secret Service protection, a congressional official told CNN, the first significant step in getting approval for the Secret Service to take over security for the rest of his campaign.

The congressional official said that the "big four congressional leaders were notified today that the Biden campaign has submitted a formal request for USSS protection." Those leaders are House Speaker Nancy Pelosi, House Minority Leader Kevin McCarthy, Senate Majority Leader Mitch McConnell and Senate Minority Leader Chuck Schumer.

"Disposition of the matter expected soon," the source said.

The next step would be to send a request to the Department of Homeland Security, which would then most likely give the go-ahead to the Secret Service to begin meetings with the campaign to discuss when the security transition would begin.

House Homeland Security Chairman Bennie Thompson said Wednesday that he raised concerns with Biden about his lack of Secret Service protection on Sunday in Mississippi -- and the former vice president told him that he would consider making a formal request.

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