WASHINGTON (MarketWatch) — Wall Street is not only the issue that most sharply divides the two leading contenders for the Democratic nomination, it is also the issue that exposes two radically different styles in politics.

And that is why, against all expectation, the Democratic primary campaign is actually a contest.

Hillary Clinton simply had no answer when Bernie Sanders turned to her in Sunday night’s debate and reminded her that “you’ve received over $600,000 in speaking fees from Goldman Sachs GS, -0.53% in one year.”

“ It is this evasiveness that distinguishes Clinton’s style of politics from the plain speaking that has drawn millions of voters this year to Sanders. ”

It was one of several remarks by Sanders about how difficult it has to be for someone who has taken money from Wall Street to be serious about regulating it. Read the transcript of the debate.

The best response Clinton could come up with was to attack Sanders for his criticism of President Barack Obama for taking donations from Wall Street, even though, she said in a brazen non sequitur, he “has led our country out of the great recession.”

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As Sanders’s surge in the polls closes his gap with Clinton in Iowa and keeps him ahead in New Hampshire — the first two states with nominating contests — it is clear that people aren’t buying Clinton’s equivocations on the subject.

And her backhanded praise of Obama did little to stop Sanders from voicing what is voters’ biggest beef about Wall Street in the wake of the financial crisis.

“Goldman Sachs [was] recently fined $5 billion,” Sanders said, as he talked about corruption in our campaign finance system. “I find it very strange that a major financial institution that pays $5 billion in fines for breaking the law, not one of their executives is prosecuted, while kids who smoke marijuana get a jail sentence.”

Sanders is calling for much bolder action against Wall Street — breaking up the biggest megabanks and restoring the Glass-Steagall separation between investment banking and commercial banking.

Clinton has tried to make the case that it was “shadow banks” like Lehman Brothers and AIG that caused the crisis, and that her proposals to add some regulation of these institutions is a better solution.

Not only does her plan blithely ignore the “too big to fail” phenomenon of the megabanks, it mischaracterizes the role of these banks in fomenting the crisis by suborning the reckless actions of shadow banks.

And it is this evasiveness that distinguishes Clinton’s style of politics from the plain speaking that has drawn millions of voters this year to Sanders.

Clinton goes on to claim, for instance, that she has a plan for Wall Street regulation “that most commentators have said is tougher, more effective, and more comprehensive.”

It is a variation on her statement in a speech earlier this month when she said that “everybody who’s looked at my proposals” thinks they’re tougher etcetera – a claim that earned her two Pinocchios from Washington Post fact checkers for making a statement that was at best only half-true.

In fact, even then, Sanders had a list of 60 economists, headed by former Labor Secretary Robert Reich and University of Texas Professor James Galbraith, who said Clinton’s plan was insufficient to address the wrongdoing on Wall Street. That list had grown to 170 economists by the time Clinton repeated her claim Sunday night.

The statement they signed fully endorsed Sanders’ plan to break up the banks and re-impose Glass-Steagall.

“Secretary Hillary Clinton’s more modest proposals do not go far enough,” the statement continued. “They call for a bit more oversight and a few new charges on shadow banking activity, but they leave intact the titanic financial conglomerates that practice most shadow banking.”

The financial system would continue to pose a serious risk to the American economy and to individual Americans under Clinton’s plan, they said. “Given the size and political power of Wall Street, her proposals would only invite more dilution and finagle,” they concluded.

Sunday’s debate also featured heated exchanges on gun control and health-care reform, with campaign claims by Clinton bringing crisp responses from Sanders.

When a moderator repeated Clinton’s remark that Sanders has been “a pretty reliable vote for the gun lobby,” the Vermont senator, citing his D- rating from the National Rifle Association, said, “I think Secretary Clinton knows that what she says is very disingenuous.”

When asked whether her claims that Sanders wants to “kill Obamacare” were fair, Clinton sidestepped the question to declare her loyalty to Obama’s health-care reform.

“Secretary Clinton didn’t answer your question,” Sanders observed to laughter. Because, he went on to say, the Clinton campaign’s claim that he wants to end Medicare or other existing government health plans “is nonsense.”

Most headlines declared Sanders the winner of Sunday’s debate, but ultimately it is Clinton’s reversion to form to rely on a politics of mendacity that is hurting her poll numbers and her chances in the primaries.