She still has to explain to the Democratic base her ties to Wall Street and the wealthy class that Sanders has effectively railed against. And she still has questions to answer about the contents of and need for a private email server while at the State Department.

But she showed important political savvy when she highlighted an important difference with the US senator from Vermont on guns, one of the nation's most emotional issues.

Sanders voted against the 1993 Brady Bill signed into law by President Bill Clinton, which required federal background checks for gun purchasers and a waiting period. Sanders also supported a 2005 measure shielding firearms makers and dealers from liability lawsuits in certain cases. Clinton, then a US senator from New York, voted no.

Asked if Sanders was tough enough on guns, Clinton did not hesitate. The woman often criticised as being emotionless showed the sort of passion supporters so badly wanted to see.

"No," she said, "not at all. It's time the entire country stood up against the NRA (National Rifle Association)."

She recalled Sanders' record. He angrily shot back that he, too, wanted sensible gun control measures. In a time when mass shootings have become numbingly routine, Clinton's clarity registered with the party base.

Sanders' explanation - that he represents a state where gun ownership is popular - was hardly the response likely to expand his constituency.

Sanders, or for that matter the three other candidates on the stage, needed this debate to emerge as a viable alternative with potential to broaden his appeal, much as Barack Obama began to do in the fall of 2007.

Obama, though, had been a US senator for less than three years and was unshackled by his legislative past. Sanders, a member of Congress since 1991, is.

Sanders has emerged as Clinton's chief challenger because of his ability to channel his passion for change, and feeling for the plight of people who still feel crushed by the placid economy, into a political movement. But the same forces that have fuelled his insurgent candidacy also limit its potential, and those limits were on display.

Sanders is a self-described "democratic socialist", a term that frightens a lot of people. He gave a vigorous, occasionally angry response when asked what that meant.

"What democratic socialism is about is saying that it is immoral and wrong that the top one-tenth of 1 per cent in this country own almost 90 per cent … own almost as much wealth as the bottom 90 per cent," he said.

But is a democratic socialist electable? Sanders said on Sunday that he was not a capitalist, and Clinton was quick to offer a distinction.

She eagerly defended capitalism, adding how "we have to save capitalism from itself". The goal, she said, was to curb the excesses of capitalism, but not reject the system that helped build a strong middle class. Sanders wants government-run health care, free college tuition and a trillion-dollar infrastructure program. At a time when many Americans are tired of expansive, intrusive government, he remains vulnerable on his notion of what socialism means.

Clinton will not escape the notion she's too close to the big money crowd. Two SuperPACs, which can raise unlimited amounts of money from special interests, support her. Sanders won't take SuperPAC funds.

She's not safe, but Clinton survived an important test.