Perhaps it was. But it's also worth questioning McCarthy's implied contention. Did the Benghazi select committee spur Clinton's decline in the polls, as McCarthy seemed to brag?
As with most poll numbers, it's hard to tell.
Over the course of the year, Hillary Clinton's polling against her Democratic opponents has certainly dipped — but it's hard to know how much of that is Benghazi and how much is Bernie Sanders. If we look instead at her favorability, we can see a longer-term decline. Since the beginning of 2013, her favorability has declined sharply; more recently, her unfavorable numbers — people who view Clinton negatively, as compiled by Huffington Post Pollster — has increased dramatically.
But, again, it's hard to know where the correlation lies. We can overlay certain key points in the development of the Benghazi investigation onto that polling, which offers some insights. There have been a variety of committee reports looking at Clinton's role in the Benghazi attacks, as well as the launch in May of last year, of the select committee.
Most of the shift in her poll numbers — particularly her unfavorable numbers — happened only this year. It appears to correlate with the revelation, first reported by the New York Times, that Clinton used a private e-mail server during her time as secretary of state — a revelation that stemmed from the select committee. Shortly afterward, there was the additional story of the Clinton Foundation's fundraising practices, which also attracted attention.
Given subsequent polling and revelations, it seems clear that the e-mail issue has been a large part of the drag on Clinton's numbers — which ties back to the select committee.
But a decline — perhaps not as steep — would also likely have happened without the committee. If we extend her favorability numbers back to the beginning of her tenure as secretary of state, we can see that the decline may extend back even further.
Part of this might be the Benghazi investigations. Part of it might be the fact that Clinton was seen as heir apparent to the Democratic nomination starting on Jan. 21, 2013. The last time Clinton ran, she saw a drop in her poll numbers as the campaign progressed — in part because Democrats bailed on her in favor of Barack Obama. In 2015, Democrats have thrown their weight behind Bernie Sanders.
Is that pushing the larger drop of her poll numbers? Are Democrats going to Sanders because of the Benghazi revelations? Both? Do people who would have backed Sanders anyway blame the e-mails? Do people now support Sanders because of them?
There's no doubt that part of the reason that the House Republicans put together the Benghazi select committee was to ensure no stone was left unturned before the 2016 cycle. The extent to which McCarthy should have taken credit for Clinton's poll drop, though, is less clear — at least from a polling perspective. From the political perspective of his desire to be speaker, it was more clearly a mistake.