Lawmakers have long abused their investigative authority for political purposes. But the effort to find Mrs. Clinton, who was secretary of state at the time of the Libya attacks, was personally responsible for the deaths has lost any semblance of credibility. It’s become an insult to the memory of four slain Americans.

Image Representative Kevin McCarthy Credit... Andrew Harnik/Associated Press

The deaths of Ambassador J. Christopher Stevens and his colleagues have been exhaustively investigated by several other congressional committees and an independent panel of experts commissioned by the Department of State. The reviews found systemic failings at the State Department. But they found no evidence that Mrs. Clinton was directly responsible for the security lapses, which, of course, is the goal of the Republicans who want to derail her presidential bid. The possibility that all those investigators have somehow missed a crucial, damning piece of evidence seems negligible.

Led by Representative Trey Gowdy of South Carolina, a former federal prosecutor, the Benghazi committee has trudged on, summoning a seemingly endless list of witnesses who have offered little new substantive information about the attacks. Since it was impaneled in May 2014, the committee has spent more than critical congressional committees, including the House Intelligence and Veterans’ Affairs Committees, which have oversight over bureaucracies with multibillion-dollar budgets.

“There’s nothing to justify the committee’s long duration or expense,” said Representative Adam Schiff, a Democrat from California who sits on the committee and has called for it to be disbanded. “We have nothing to tell the families and nothing to tell the American people.”

Mrs. Clinton is scheduled to testify before the committee on Oct. 22. The hearing will give Republicans another chance to attack the credibility and trustworthiness of the leading Democratic presidential candidate. It will do nothing to make American embassies abroad safer or help the relatives of the four killed in Libya.