Bernie Sanders sat down for an interview with The Washington Post over the weekend. This is, by far, the most important thing he said in it:

I do not believe that you can get huge speaking fees from Goldman Sachs and then with a straight face tell the American people that you’re prepared to do what is necessary to take on the greed and illegal behavior on Wall Street. I don’t think people think that passes the laugh test. . . . Why do special powerful interests give you money? Are they dumb? I don’t think so.

That, in the space of 71 words, is Sanders's strongest closing argument against Clinton in this week leading up to the Iowa caucuses. It not only casts the difference between Clinton as consummate insider and Sanders as outsider-in-chief but also effectively raises the central question at the heart of this race: Can someone who has played the game for as long and as well as Clinton possibly be able to fundamentally change that game?

Then there's the more visceral hit that bringing up Clinton's speeches gets Sanders. The idea that Clinton was paid hundreds of thousands of dollars by Goldman Sachs and lots of other major corporations — here's a full-ish rundown of the $11 million Clinton earned from speeches in 2014 and the first three months of 2015 — suggests a level of access and elitism that many people will blanch at. Clinton got paid $200,000 for a single speech to Goldman Sachs; the average median family income in Iowa is just more than one-quarter of that ($53,712).

Then there's the fact that Clinton hasn't, really, found a good response to this line of attack from Sanders.

In the fourth Democratic debate earlier this month, Sanders went right at Clinton on the issue. Here's, in part, what he said:

The leader of Goldman Sachs is a billionaire who comes to Congress and tells us we should cut Social Security, Medicare and Medicaid. Secretary Clinton — and you're not the only one, so I don't mean to just point the finger at you, you've received over $600,000 in speaking fees from Goldman Sachs in one year. 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.

Clinton's response? "Senator Sanders, you're the only one on this stage that voted to deregulate the financial market in 2000, to take the cops off the street, to use Governor O'Malley's phrase, to make the SEC and the Commodities Futures Trading Commission no longer able to regulate swaps and derivatives." Which, of course, is not even close to a direct rebuttal of Sanders's claim that Goldman was buying access to Clinton or, more damagingly, that she was selling it.

On Friday in New Hampshire, a reporter for the Intercept asked Clinton whether she would release the transcripts of the speeches she gave to Goldman Sachs. She laughed.

Then on Sunday, Clinton was on "Meet the Press." MTP moderator Chuck Todd asked Clinton this key question: "Why do you think one of these big banks paid you over $200,000 for a speech?"

Here's her response:

Well, look, I gave speeches to a wide array of groups, from health-care groups to auto dealers and many, many more. And I think what they were interested in, because what we talked about was the world. Coming off of four years as secretary of state, in a complicated world, people were interested in what I saw, what I thought; they asked questions about the matters that were on their mind, a lot of interest in the bin Laden raid, how such a tough decision was made and what I advised the president. You know, I think Americans who are doing business in every aspect of the economy want to know more about the world. I actually think it's a good conversation to be having.”

"You don't think they expect anything in return?," Todd interjected. "Absolutely not," Clinton responded.

There's so much wrapped up in Sanders's hit on Clinton's speaking fees. Her insider-ness. The idea — totally unproven and likely impossible to prove — that she was, in part, selling access to well-heeled corporations. Her perceived flexibility on issues. Her inability to simply say some things are right while others are wrong. And so on and so forth.

For Sanders, the speaking fees attack encapsulates all of his strengths and Clinton's weaknesses into a tidy package that is easily understood by almost anyone.

In short, you can expect to hear lots more about Goldman Sachs before this week is out, Iowa Democratic voters.