To judge by President Trump's public statements (particularly his Twitter feed), you might think that what most concerns him about Robert Mueller's investigation is the possibility that anyone would believe that he and his campaign colluded with Russia in the 2016 election. "As Paul Manaforts lawyer said, there was 'no collusion,'" he tweeted, citing an unimpeachably objective source. "Also, there is NO COLLUSION!" read another tweet.

Trump is plainly sensitive about anything that might cast doubt on the legitimacy of his election victory. But that's not what he's really afraid of. The real threat Mueller poses, the thing that's probably keeping Trump up at night, is what he might find out about the president's finances. If Mueller follows the money — and he certainly will — it could lead to all sorts of interesting places.

To begin, it's important to understand that although Mueller's investigation is concerned with possible collusion between the Trump campaign and Russia, that investigation can expand anywhere he encounters evidence that crimes might have been committed. Since Trump has copious connections to Russian money, it was inevitable that Mueller would begin probing his finances.

But that's not what Trump insists. Consider this exchange from an interview he did with The New York Times in July, when he suggested that if Mueller started looking into his finances it would be grounds to fire him:

SCHMIDT: Last thing, if Mueller was looking at your finances and your family finances, unrelated to Russia — is that a red line? HABERMAN: Would that be a breach of what his actual charge is? TRUMP: I would say yeah. I would say yes. By the way, I would say, I don't — I don't — I mean, it's possible there's a condo or something, so, you know, I sell a lot of condo units, and somebody from Russia buys a condo, who knows? I don't make money from Russia. In fact, I put out a letter saying that I don't make — from one of the most highly respected law firms, accounting firms. I don't have buildings in Russia. They said I own buildings in Russia. I don't. They said I made money from Russia. I don't. It's not my thing. I don't, I don't do that. Over the years, I've looked at maybe doing a deal in Russia, but I never did one. [The New York Times]

Despite the rather uncharacteristic bit of modesty, Trump is right that he never invested in Russia. But a whole lot of Russians invested in him — more than just a condo here and there. In fact, Russians, including a large number of oligarchs and mobsters, have bought hundreds of millions of dollars worth of Trump properties over the years, in many cases apparently as a means of laundering money. Trump has also had Russian associates and Russian investors with questionable ties who have joined with him on troubled projects like Trump Soho. So there are a lot of threads for a prosecutor to start tugging on.

But more broadly, once Mueller begins looking into Trump's books, there's no telling what he'll find. If you have even a passing familiarity with Trump's business career, you know that it's full of scams like Trump University, vendors he refused to pay, labor law violations, and deals he managed to slip out of while leaving others holding the bag.

All of it was investigated only perfunctorily by the press during the 2016 campaign: an article here or there explaining some shady deal from Trump's past, with little or no follow-up from the rest of the media. For instance, did you know that Trump once had to pay a $750,000 fine in an insider trading case? I'll bet you didn't. For any other candidate it might have been disqualifying, but for him it was barely worth noticing. Did you know that Trump had substantial dealings with the mafia in New York, which was heavily involved in the construction industry? You probably didn't hear about that either.

Much of that may be in the past, but we don't even know what Trump is involved in right now. Because the Trump Organization is a private company, its details are not open to public view — and in fact, the company is really a series of hundreds of smaller companies, partnerships, and arrangements, a kind of gigantic financial rat king in which the component parts send money flowing to Donald Trump from all over the world.

And think about the risk Trump took to keep his tax returns secret, in defiance of half a century of political tradition. He knew that he'd get significant criticism for the decision and it would lead to lots of uncomfortable questions, but he was adamant that no one would get to see those returns.

But Robert Mueller can. He can subpoena them whenever he decides it's necessary. For all we know he already has. And if it's relevant to the investigation and it appears that there might have been crimes committed by someone, he can go deeper and deeper, an examination Trump has never been subjected to before. Mueller made a point of hiring a number of lawyers for his team who have expertise in money laundering and other financial crimes; they helped build the case against Paul Manafort, but they'd also know what to look for in Donald Trump's financial web.

Trump spent a lifetime crafting scams and schemes, ways to get over on people and skate away without taking responsibility. He dealt with an endless series of shady characters as he built his fortune; as he said last year without any hint of shame, "My whole life I've been greedy, greedy, greedy, I grabbed all the money I could get, I'm so greedy."

Now raise your hand if you think that in all of his decades as a businessman, Donald Trump was careful never to break any laws or run afoul of any ethical standards, and Mueller will find nothing in Trump's finances that could be trouble for him if revealed. Anyone? I didn't think so.