A gaffe, in Michael Kinsley's famous formulation, is "when a politician tells the truth — some obvious truth he isn't supposed to say."

On Tuesday night, Rep. Kevin McCarthy — the overwhelming favorite to succeed John Boehner as speaker of the House — made Kinsley proud.

During an appearance on Sean Hannity's Fox News show, McCarthy bragged that the House GOP's investigation into the Benghazi attack had made Hillary Clinton's poll numbers plummet. Furthermore, he presented it as part of a "strategy to fight and win," rather than a nonpartisan effort to find the truth.

Here's what he said, via Roll Call:

What you’re going to see is a conservative speaker, that takes a conservative Congress, that puts a strategy to fight and win. And let me give you one example. Everybody thought Hillary Clinton was unbeatable, right? But we put together a Benghazi special committee. A select committee. What are her numbers today? Her numbers are dropping. Why? Because she’s untrustable. But no one would have known that any of that had happened had we not fought to make that happen.

"I give you credit for that," Hannity responded. "I'll give you credit where credit is due."

The Select Committee on Benghazi's investigation has long appeared partisan

Several investigations into the Benghazi events have already wrapped up without findings of serious wrongdoing. Yet the House Select Committee on Benghazi has kept at it — and has long appeared unusually focused on Hillary Clinton, and even on a member of her circle who wasn't even in the administration.

In June, the committee interviewed Sidney Blumenthal — a Clinton friend who emailed her advice and information while she was secretary of state, but had no particular useful knowledge about Benghazi — for nine hours. Vox's Jonathan Allen wrote then that they had "jumped the shark," finally stripping away "any pretense that they are more interested in the Benghazi attack than in attacking Hillary Clinton."

Still, the public line from Republicans was that they were merely trying to find the truth about what happened. Without this insistence that the investigation was nonpartisan, it's somewhat tougher for the media and the public to take them seriously.

So McCarthy's flub has overjoyed Democrats long hoping to discredit the investigation. Groups like Media Matters ensured the video was distributed far and wide. Brad Woodhouse of the pro-Clinton group Correct the Record harrumphed to David Weigel, "We have been saying for years that Republicans were exploiting the deaths of four Americans for political gain. Kevin McCarthy just admitted it. Disgraceful." And on Wednesday, Rep. Eliot Engel (D-NY) called for the committee to be shut down entirely.

Comments like McCarthy's — even if they were well understood by most politicos in private — hurt the Select Committee on Benghazi's credibility among people who weren't already biased against it. Its more ambiguous findings, and the more opinionated judgments it issues, will be viewed with more skepticism.

Still, if the committee finds something real, it will be taken seriously. It's already hugely changed the 2016 race by setting in motion the scandal over Hillary Clinton's use of a personal email system for State Department business. That was a genuinely newsworthy revelation, even if it was discovered by a partisan source. If the committee manages to find something else similar, what McCarthy said about it won't really matter.

VIDEO: Hillary Clinton on her use of private email