The demand follows a Senate request months earlier for the campaign committee to preserve documents.
Dozens of former staffers are expected to be contacted in the coming days to make sure they are aware of what they are required to produce and how to submit those documents, the people added.
The letter was signed by Sen. Richard Burr (R-N.C.), the Senate committee’s chairman, and Sen. Mark R. Warner (D-Va.), the committee’s ranking Democrat. Spokespeople for Burr and Warner declined to comment.
The request to Trump’s political operatives represents the first time that Trump’s official campaign structure has been drawn into the Senate committee’s ongoing bipartisan investigation. That investigation is separate from the federal probe being led by the Justice Department’s special counsel, former FBI director Robert S. Mueller III.
In recent months, several Trump campaign associates, such as Roger Stone and Carter Page, have been contacted by Senate investigators, but the campaign itself had not been asked to preserve and produce materials.
Trump’s campaign committee is now led by former deputy campaign manager Michael Glassner and John Pence, a nephew of Vice President Pence. It is based in New York at Trump Tower. Glassner did not immediately respond to a request for comment.
A White House spokesperson had no immediate comment.
John Wagner and Karoun Demirjian contributed to this report.