The Senate letter accuses the F.B.I. of relying largely on unverified information produced for the Democratic National Committee and Hillary Clinton’s presidential campaign to convince a federal judge of the need to eavesdrop on Mr. Page as a possible Russian agent.

The letter said the Justice Department’s initial application to the Foreign Intelligence Surveillance Court to spy on Mr. Page, filed in October 2016 after he had left the Trump campaign, “appears to contain no additional information corroborating the dossier allegations” posed by Christopher Steele, a former British intelligence agent who had been working with a research firm paid by the Democrats. That court approved the surveillance for 90 days, and renewed it three times.

Republicans have said that the so-called Steele dossier was the main justification for the F.B.I.’s investigation into possible Russian influence over the Trump campaign, casting doubt on the inquiry’s legitimacy. Democrats on the House Intelligence Committee have said that Republicans are distorting the roots of the inquiry in an effort to bolster Mr. Trump.

Last week, Mr. Trump declassified a memo by Republicans on the House Intelligence Committee that leveled similar allegations to those posed by Mr. Grassley and Mr. Graham. Mr. Trump, who had a brief window to block the memo’s disclosure on national security grounds, did not ask that any of its substance be redacted.

Administration officials said that Mr. Trump had read the Democratic memorandum, which seeks to undermine Republican claims that top law enforcement officials had abused their powers when they sought a warrant to wiretap Mr. Page. But Sarah Huckabee Sanders, the White House press secretary, declined to say whether the president intended to handle the matter the same way he did the Republican memo. That document also contained accusations of F.B.I. bias.