After running a congressional oversight committee like a Republican opposition research shop for more than two years, Representative Trey Gowdy appears to be gearing up for the finale. Democrats on the Select Committee on Benghazi expect that a final report will drop soon, just as Hillary Clinton appears poised to clinch the Democratic nomination.

If things had gone his way, Mr. Gowdy, a former federal prosecutor, would have found a way to torpedo Mrs. Clinton’s presidential ambitions. After all, Republican lawmakers have admitted that this is precisely what they set out to do.

But things have not gone well for Mr. Gowdy, who has run the investigation with the dexterity and grace of a blindfolded toddler swinging at a piñata. Having pored over reams of documents, grilled Mrs. Clinton in an 11-hour session in October and hauled in more than 100 people for interviews, the Republicans seem to have come up with nothing.

In recent months, Republicans on the committee have pestered the Pentagon to track down potential witnesses who might have damning things to say about Washington’s response to the attack on American government facilities in Benghazi, in eastern Libya, on Sept. 11, 2012, when Mrs. Clinton was secretary of state. They include a man who identified himself as a military mechanic in an intriguing Facebook post, and “John from Iowa,” a person who claimed to be a drone operator who had called into a right-wing radio talk show.