Sen. Bernie Sanders (I-Vt.) might have found a new way to cast some doubt: Highlighting her preference for regime change in the Middle East. Specifically, he sought to tie Clinton's preference for toppling a dictator in now-troubled Libya to her plan for fighting the Islamic State in Syria, where Clinton wants to overthrow Syrian President Bashar al-Assad.
"I worry ... that Secretary Clinton is too much into regime change and a little bit too aggressive without knowing what the unintended consequences might be," Sanders said.
And later: "So I think secretary Clinton and I have a fundamental disagreement. I'm not quite the fan of regime change that I believe she is."
The logic here is that Libya is a trouble spot in Clinton’s foreign policy record and that focusing on regime change makes her look too aggressive. As secretary of state, Clinton advocated for and eventually helped oversee U.S. military intervention there in 2011 that eventually led to the death of Libyan dictator Moammar Gaddafi.
Today, Libya is in a chaotic civil war, is a potential breeding ground for terrorists, and four Americans, including a U.S. ambassador, are dead after attacks in Benghazi in 2012.
Yet, for really one of the first times Saturday, Clinton was forced to play defense when it comes to her leadership on Libya. (Remember that 11-hour congressional hearing this fall on the Benghazi attacks?)
Her response? Clinton said the Libya situation is not perfect, but she put much of the blame on forces outside her control, like political unrest from the Arab Spring. While she said there are no easy solutions in the Middle East, she maintained that toppling Assad is necessary to defeat the Islamic State, since the chaos he's creating is giving space for the terrorist group to grow.
"The reason we are in the mess we're in, that ISIS has the territory it has, is because of Assad," she said, using an acronym for the Islamic State.
But isn't that what the Clinton tried to do just a few years ago?, Sanders asked, adding if it were up to him, removing Assad would take a clear back seat to efforts against the Islamic State.
His argument is that regime change is the quintessential example of America's act-first, think-later approach to problem-solving abroad that got America drawn into the Middle East in the first place.
"Yes, we could get rid of Saddam Hussein, but that destabilized the entire region," he said. "Yes, we could get rid of Gaddafi, a terrible dictator, but that created a vacuum for ISIS. Yes, we could get rid of Assad tomorrow, but that would create another political vacuum that would benefit ISIS. So I think, yeah, regime change is easy, getting rid of dictators is easy. But before you do that, you've got to think about what happens the day after."
In other words, Clinton's approach didn't work then, and there's little reason to believe it will work now.
Clinton's closest challenger isn't the only 2016 candidate to question whether regime change is the right path in Syria. The other candidate on stage Saturday, former Maryland governor Martin O'Malley, agreed with Sanders that it might not be the way to go. And on the Republican side, some 2016 candidates, including GOP front-runner Donald Trump, wondered in their debate Tuesday whether letting Assad stay in power might be okay or even beneficial, despite his having used chemical weapons on his own people.
Still, Sanders's line of attack isn't perfect. As Clinton noted in the debate, Sanders actually joined with all of his colleagues in the Senate in 2011 in supporting a resolution calling for "an orderly, irreversible transition to a legitimate democratic government in Libya."
And Sanders has struggled recently to address the Islamic State with the kind of thorough detail that Clinton has -- with his campaign even going so far as to request reporters not ask him about it.
But with Clinton so far ahead of her competitors on foreign policy, questioning her support for regime change might be the best shot any of her opponents have at trying to slow her down.