Democratic U.S. presidential candidate and former Secretary of State Hillary Clinton speaks during the second official 2016 U.S. Democratic presidential candidates debate in Des Moines, Iowa, November 14, 2015.

Clinton's rivals wasted no time jumping on the comments as factually wrong and deeply insensitive. Former Maryland Gov. Martin O'Malley on Monday ripped Clinton, calling it "shameful" that the former secretary of state and New York U.S. senator linked 9/11 to her donations from Wall Street.

Pressed by Sen. Bernie Sanders about her strong financial support from Wall Street, Clinton said: "'I represented New York, and I represented New York on 9/11 when we were attacked … Where were we attacked? We were attacked in downtown Manhattan, where Wall Street is. I did spend a whole lot of time and effort helping them rebuild. That was good for New York. It was good for the economy and it was a way to rebuke the terrorists who had attacked our country.'"

Hillary Clinton had a mostly strong debate, especially on national security in the wake of the Paris attacks. But she gave opponents in both parties one huge opening with her remarks on her support from Wall Street and the Sept. 11, 2001, terrorist attacks.

"I thought that moment, frankly, was pretty shameful. I don't believe that the people watching were applauding the notion that Secretary Clinton was pumping out this smokescreen and wrapping herself in the tragedy of 9/11," O'Malley said on CNN's "New Day" program. "I don't think they saw that as something appropriate to do, to mask her coziness and her closeness to Wall Street and all of the architects of the crash of 2008."

Republicans got in on the act as well. RNC chair Reince Priebus Tweeted: "@HillaryClinton, you reached a new low tonight by using 9/11 to defend your campaign donations."

You can bet ad makers will have a field day with the remarks, using them to show that Clinton will say just about anything to win. And it did not get any better later in the debate when Clinton was asked to respond to a Twitter question from someone who said it was inappropriate to link Wall Street donations to 9/11.

Clinton's response? "I am sorry if whoever tweeted that had that impression." Saying it is the questioner's fault for having that "impression" is the worst kind of non-apology.

The gaffe is not likely to seriously threaten Clinton's march to the Democratic nomination. But it does raise some serious questions.

The first is: How could she not have had a better prepared answer?

O'Malley and Sanders have made Clinton's Wall Street ties and positions the key pillar of their assault on her. Clinton is known for meticulous preparation.

She could have easily just recited her own support for stronger bank regulation and breaking up banks if they pose a threat and said Wall Street should expect no favors from her in the White House.

But she didn't do that. It seems like the 9/11 remark was in fact a planned part of her response. And if that's the case … huh? Who in the campaign thought that was a smart idea?

Forget the fact that much of "Wall Street" had already moved to midtown Manhattan by 2001. The fact is that Wall Street's support for Clinton has little to nothing to do with her response to the attacks. Do they appreciate that she was helpful? Sure.

But mostly they view her as a far more palatable choice for president than Sanders and O'Malley. That's in large part because she does not support a re-imposition of the Glass-Steagall wall between traditional and investment banking. It's also because they think she would be better for the economy overall than Sanders or O'Malley or many of the potential Republican nominees.