For months, Carter Page, the former Trump campaign adviser who was under government surveillance as part of the Russia investigation, has been shunned by Republicans and dismissed by the White House, which portrayed his campaign stint as inconsequential.

But now Mr. Page is the linchpin in a conservative effort to discredit the F.B.I. and the special counsel inquiry. He is at the center of a divisive memo written by Republican committee staff members that was released on Friday and accuses law enforcement officials of abuses in obtaining a warrant to surveil Mr. Page in 2016.

The memo falls short of the case that some Republicans promised — that the document would show bias against Mr. Trump by investigators in opening the Russia inquiry and possibly undercut the investigation by the special counsel, Robert S. Mueller III.

But for the past year, Mr. Page himself has been pitching that narrative to journalists, politicians, investigators and almost anyone who will listen. Though Mr. Trump’s allies have repeatedly sought to dismiss him as a bit character in the 2016 campaign, Mr. Page’s role could now be political fodder in the president’s efforts to discredit Mr. Mueller’s inquiry.