Bernie Sanders has spent months hammering Hillary Clinton for accepting millions of dollars in speaking fees from Wall Street in the runup to her presidential bid, while refusing to release transcripts of the speeches to the public.

Sanders has done it so many times, he's even come up with a favorite joke:

"I am going to release all of the transcripts of the speeches that I gave on Wall Street behind closed doors," Sanders said again during a Democratic debate Thursday night on CNN. "There were no speeches."

The crowd roared.

On its simplest level, this resonates because of a basic contrast. While Clinton's cashed in on Wall Street, Sanders has been plotting political revolution — an image at the center of his campaign. But the issue is so powerful because, much more so than the average Sanders jab, it reminds a large swath of Democrats of their biggest doubt about Clinton: her judgment.

Why Clinton struggles to parry the speech attack

At its most obvious, the speech issue fits Sanders's criticism of both Clinton and many Democratic officials as being far too dependent on the financial contributions of powerful businesses. Highlighting her speaking fees from the banking industry is a simple, clear illustration of this underlying narrative.

For months, Clinton has struggled to find a compelling response to this line of attack. Asked why she took the cash during a debate in early February, Clinton said: "I don't know. That’s what they offered."

On Thursday, she tried to instead use her policy positions as a shield, arguing that she "stood up against the behaviors of the banks" as a senator.

"I called them out on their mortgage behavior," she said. "I also was very willing to speak out against some of the special privileges they had."

But Sanders sees this as nonresponsive to his basic question. The very idea that Clinton could both crack down on the big banks and take their money at the same time strikes him as straight-up laughable. Anyone they were willing to shower with so much cash can't have been perceived as a real threat to their interests.

"Hillary Clinton 'called them out,'" Sanders responded, sarcastically. "They must've been really crushed by this. Was that before or after you received huge sums of money after giving speaking engagements behind closed doors? They've must've been very upset."

The reality may be more complicated than Sanders suggests. After all, most campaign finance experts don't think the problem with the current system is that politicians transactionally reward their donors with policies suited to directly benefit them.

The bigger problem: Clinton's judgment

But the attack remains an effective one for another reason altogether: It feeds into broader doubts about Hillary's decision-making.

In 2007 and 2008, the press was clamoring for the Clinton Foundation to publicly release its list of donors. When Hillary became secretary of state in 2009, Bill Clinton cashed in by jacking up his speaking fees — earning $500,000 from a Russian investment bank and $750,000 for an address to a telecom conference in China, according to ABC News.

This kind of behavior clearly creates political liabilities for the couple, but they spent years determined to plow ahead with the buckraking, regardless of its impact on their standing in the polls. That's their choice to make, on some level, but a presidential nominee is the de facto leader of an entire political party, which makes a would-be nominee's judgment about political choices and trade-offs everyone's business.

This is why you don't need to be a Bernie activist or an Occupy Wall Street protester to be troubled by Clinton's speaking fees. It's not so much that they prove that Clinton is in Lloyd Blankfein's pocket but that they magnify a pattern of questionable decisions. How did she not see this would be a political poison pill? How did nobody in her inner circle warn her this would be a bad idea? It's one thing, after all, to run a big political risk in order to try to achieve an important policy goal. It's another thing entirely to do it for personal financial gain.

Fantasy questions: Sen. Clinton, doesn't it at least create the appearance of corruption to accept $675,000 from Goldman Sachs? Why do it? — Ezra Klein (@ezraklein) April 15, 2016

Clinton clearly has a tremendous number of strengths as a candidate. She has an unparalleled command of policy details, a set of practical and thoughtful proposals, and a deep knowledge of foreign policy. Asked whether she has the right experience to be president, 66 percent of Americans say yes.

Clinton seems genuinely convinced that the money from Goldman will have no impact on the policies her administration pursues. And maybe she's right. But the fact that she didn't realize — or didn't care — that many voters would disagree suggests a gap in her thinking that could prove a dangerous distraction in the White House.