House Majority Leader Kevin McCarthy commented on Fox News on the Select Committee on Benghazi, suggesting its investigation brought Hillary Clinton's poll numbers down. Washington Post reporter Elise Viebeck explains why McCarthy's comment ended up as a win for Clinton. (Alice Li/The Washington Post)

When House speaker-in-waiting Kevin McCarthy claimed credit for the decline in Hillary Clinton's poll numbers because House Republicans had formed a congressional committee investigating the attacks in Benghazi, Libya, I knew it was a bad political mistake. I may have underestimated just how bad.

Consider how Clinton responded to a question about her private e-mail server during a New Hampshire townhall hosted by NBC's "Today" show co-host Savannah Guthrie on Monday:

"This committee was set up, as they have admitted, for the purpose of making a partisan political issue out of the deaths of four Americans," she said. "I would never have done that, and if I were president and there were Republicans or Democrats thinking about that, I would have done everything to shut it down."

Throughout the interview, Clinton referenced McCarthy's comments — made during a contentious interview with Fox News Channel's Sean Hannity — as a means to do something she's been unable to do ever since the e-mail story broke in March: Play offense.

[Watch Clinton and see for yourself]

She's impassioned and right on the edge of anger. Anger is, of course, not always a good thing for politicians.  But in this case, it shows Clinton at her best: Partisan, passionate and convincing.

Clinton has tried the political witch hunt tactic before — without success. Here's what Clinton said at an Iowa Democratic dinner back in August about the issue:

You know what?  It’s not about e-mails or servers either.  It’s about politics.  I will do my part to provide transparency to Americans.  That’s why I’m insisting 55,000 pages of my e-mails be published as soon as possible.  I’ve even offered to answer questions for months before Congress.  I’ve just provided my server to the Justice Department.  But here’s what I won’t do: I won’t get down in the mud with them.  I won’t play politics with national security or dishonor the memory of those who we lost.  I won’t pretend that this is anything other than what it is – the same old partisan games we’ve seen so many times before.

At the time, I wrote a piece headlined: "Hillary Clinton is trying to make the e-mail controversy political. But, really, it isn’t." (Republicans liked that piece then — and now; John Boehner tweeted it out  Sunday night.) My conclusion then was that while the Benghazi committee was aware that Clinton was the frontrunner for the Democratic nomination, the GOP leadership had been very careful to frame it as simply a fact-finding effort that happened to include the former secretary of state.

McCarthy, of course, changed all that with his comments to Hannity. What McCarthy said doesn't mean that the who, what, when, where and why of Clinton's exclusive use of a private e-mail server while at the State department isn't still an issue for her. What it does mean though is that she now has a way to change the subject — or at least the terrain on which she is fighting.

For months and months, all Clinton could do is say how she didn't do anything legally wrong. Then, eventually, she apologized for setting up the server. There was no offensive plan or strategy that worked. That all changed with McCarthy's comments.  Now, Clinton can do exactly what she did in New Hampshire n Monday morning: Quickly acknowledge her error then turn to the "real" issue, which is how Republicans are exploiting the deaths of four Americans for their own political gain.

That narrative — coupled with Clinton's passion for it — is a winner, or as close as she can get to it on an issue that has caused tons of problems for her campaign. Tonally, Clinton seemed more in her comfort zone during Monday's townhall than I have seen her in months and months.

Put simply: She's at her best when she is taking the fight to someone. Think of how much better a candidate Clinton was from March 2008 until June 2008 as the underdog than she ever was in the "front-runner" role in that race. She is an offensive player, not a defensive one.

It's possible that this past five days or so is simply a blip in the broader context of the campaign and the Clinton-plays-defense storyline will reassert itself. But, the Clinton team thinks that McCarthy has handed them a massive political gift, and I tend to agree.