The morning after a debate, it's natural to focus on the most dramatic moments. But in the case of Thursday night's Clinton-Sanders showdown, the most significant exchange was arguably one that featured almost no drama. It's a dog that didn't bark: a moment where it initially looked like Sanders was going to hammer Clinton on her Achilles heel — personal, professional, and financial ties to Wall Street — but ended up retreating into generalities.

And what's really striking about it is that it wasn't a blunder or a missed opportunity on his part. He wasn't able to blast away at Clinton's weak spot because she very effectively covered it with a human shield named Barack Obama — forcing Sanders to choose between slamming a president who has a 90 percent approval rating among Democrats and abandoning his key argument against Clinton.

It came about midway through the domestic portion of the debate, when Sanders — who'd been rambling a bit — started to close in on his view that Clinton is hopelessly compromised by a system of money and power in Washington.

"Secretary Clinton's Super PAC, as I understand it, received $25 million dollars last reporting period, $15 million dollars from Wall Street," he said. "Our average contribution is $27 dollars; I'm very proud of that."

Sanders was clearly winding up to throw some kind of punch, but before he could, moderator Gwen Ifill said, "Sen. Sanders, are you saying—" and then Clinton cut her off and launched her move.

I debated then-Sen. Obama numerous times on stages like this, and he was the recipient of the largest number of Wall Street donations of anybody running on the Democratic side ever. Now, when it mattered, he stood up and took on Wall Street. He pushed through, and he passed the Dodd-Frank regulation, the toughest regulations since the 1930s. So, let's not in anyway imply here that either President Obama or myself, would in anyway not take on any vested interested, whether it's Wall Street, or drug companies, or insurance companies, or frankly, the gun lobby to stand up to do what's best for the American people.

On its face, this isn't an amazingly strong argument. "Barack did it too," as we all remember from second grade, is not a real defense against charges of misconduct. But in the context of this particular Democratic primary, it's a daring gambit. Rather than directly defend herself against the charge of having been corrupted by Wall Street campaign contributions, Clinton is taking Obama hostage.

She is inviting Sanders to offer the argument that he knows — and she knows, and every journalist who follows internecine Democratic Party debates knows — many of his earliest and most fervent supporters believe: The Dodd-Frank bill is a hopelessly compromised and ineffectual piece of legislation that passed precisely because the Obama administration and many other senior Democrats are financially and intellectually captured by Wall Street. The left critique of Obama would go on to note that not only has Obama taken big money from Wall Street, but a huge number of early members of his administration's economic team now work for financial services companies, and he's continued to appoint new people to key regulatory posts who have experience in private industry.

But Sanders didn't have the guts to take the shot at Obama. So instead he said:

Let's not insult the intelligence of the American people. People aren't dumb. Why in God's name does Wall Street make huge campaign contributions? I guess just for the fun of it; they want to throw money around. Why does the pharmaceutical industry make huge campaign contributions? Any connection maybe to the fact that our people pay the highest prices in the world for prescription drugs? Why does the fossil fuel industry pay — spend huge amounts of money on campaign contributions? Any connection to the fact that not one Republican candidate for president thinks and agrees with the scientific community that climate change is real and that we have got to transform our energy system? And when we talk about Wall Street, let's talk about Wall Street. I voted for Dodd-Frank, got an important amendment in it. In my view, it doesn't go anywhere near far enough. But when we talk about Wall Street, you have Wall Street and major banks have paid $200 billion in fines since the great crash. No Wall Street executive has been prosecuted.

The problem here is that now Sanders is on consensus ground. Clinton agrees with Sanders that the campaign finance system should be reformed. She agrees with Sanders that we need more restrictions on carbon dioxide emissions. And she agrees with Sanders that we need to go beyond the rules in the Dodd-Frank bill. That's just the common ground of all mainstream Democrats, who are all facing the problem that bills need majority support in the House of Representatives to become law.

Now that we're within this broad zone of agreement, Clinton delves into her absolutely favorite terrain — policy details:

Let's just follow up on this, because, you know, I've made it very clear that no bank is too big to fail, no executive too powerful to jail, and because of Dodd-Frank, we now have in law a process that the president, the Federal Reserve, and others can use if any bank poses a systemic risk. I think that's a major accomplishment. I agree, however, it doesn't go far enough, because what it focuses on are the big banks, which Sen. Sanders has talked about a lot, for good reason. I go further in the plan that I've proposed, which has been called the toughest, most effective, comprehensive plan for reining in the other risks that the financial system could face. It was an investment bank, Lehman Brothers, that contributed to our collapse. It was a big insurance company, AIG. It was Countrywide Mortgage. My plan would sweep all of them into a regulatory framework so we can try to get ahead of what the next problems might be. And I believe that not only Barney Frank, Paul Krugman, and others have said that what I have proposed is the most effective. It goes in the right direction. We have Dodd-Frank. We can use it to break up the banks, if that's appropriate. But let's not kid ourselves. As we speak, there are new problems on the horizon. I want to get ahead of those, and that's why I've proposed a much more comprehensive approach to deal with all of these.

Sanders then manages to get the last word in, and even ends with the audience applauding:

Let me, you know, again, respectfully disagree with Secretary Clinton here. When you have three out of the four largest financial institutions in this country bigger today than they were when we bailed them out because they were too big to fail, when you have six financial institutions having assets equivalent to 58 percent of the GDP of America, while issuing two-thirds of the credit cards and a third of the mortgages, look, I think if Teddy Roosevelt were alive today, that great trust-buster would have said break them up. I think he would have been right. I think he would have said bring back a 21st-century Glass-Steagall legislation. I think that would have been right, as well. That's my view.

In the moment, I think this must have felt like a win for him. He evaded Clinton's efforts to get him to attack Obama, and ended on a crowd-pleasing anti-bank stem-winder.

But strategically, he lost. Which is to say he undermined the core rationale for his campaign. If the question on the table is whose financial regulatory legislation program should be submitted to be dead on arrival in Congress, then the tie goes to Clinton. She's ahead in national polls already, she's better known, she's endorsed by way more politicians and advocacy group leaders, she's conversant in a wider range of issues, and she's a stronger candidate in the fall. The Sanders campaign has answers on all those points, of course, but none of them are core to his candidacy.

The core of his candidacy is that the Democratic Party needs a leader who is not part of the corrupt system of money in politics — a system whose pernicious effects are wide-ranging but most clearly visible in the kid gloves treatment Wall Street receives.

If you believe that, then you have a very good reason to vote Sanders, even if you think there's a lot to admire about Hillary Clinton and her career. But if you believe that, then you have to believe that Obama is also part of the corrupt system. That's certainly what a lot of Sanders fans I know in Washington and in the media world think. But Obama is very popular among Democrats, and Sanders doesn't want to attack him directly — which prevents him from properly stating his case against Clinton.