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“Follow the money.”

Those three words – which first emerged in the Watergate scandal and have become the iconic call to arms for investigative journalists everywhere – tell you exactly why the news that Allen Weisselberg was granted immunity in the investigation into Michael Cohen is such a gigantic deal.

Weisselberg is the chief financial officer of the Trump Organization – but he’s so much more than that. Hired first by Trump’s father, Weisselberg has been the money man in Trump’s orbit for decades. And, he didn’t just handle finances for the Trump Organization but also for Trump’s personal accounts.

Which brings us to why he was granted immunity by prosecutors in the Southern District of New York in their pursuit of Cohen who pleaded guilty to eight felonies earlier this week. Two of those eight counts dealt with campaign finance violations tied to payments made to two women – Stormy Daniels and Karen McDougal – who alleged that they had affairs with Trump. Cohen engineered payments to those women totaling almost $300,000 and, in the plea agreement, he said that he did so at the “direction” of Trump.

Trump, via his Twitter feed, has made clear that he believes himself to be entirely innocent because he gave Cohen the money to pay the women from his personal account not the 2016 campaign account. “Michael Cohen plead guilty to two counts of campaign finance violations that are not a crime,” Trump tweeted earlier this week.

He’s wrong about that as the New York Times made clear in a piece that published Thursday night. Here’s the key passage:

“The Trump Organization recorded the reimbursement as a legal expense. But Mr. Cohen, Mr. Trump’s longtime fixer, said on Tuesday that he paid Ms. Clifford, better known as Stormy Daniels, to buy her silence during the 2016 campaign. Federal prosecutors have said the reimbursement payments were for sham legal invoices in connection with a nonexistent retainer agreement. Mr. Cohen, who pleaded guilty to federal campaign finance charges, did no legal work in connection with the matter, prosecutors said.”

Which is where Weisselberg comes in. As the man in charge of the money, he was/is uniquely positioned to explain to prosecutors how Cohen was paid back for the $130,000 he gave to Daniels (real name: Stephanie Clifford). Rudy Giuliani, Trump’s lawyer, broke news on that front in May when he made clear that Trump had repaid Cohen the money – despite Trump’s denials a month earlier that he had no idea about the payment or where the money for it came from.

“When I heard of Cohen’s retainer for $130,000, he was doing no work for the President,” Giuliani told conservative pundit Sean Hannity in May. “I said, ‘Well, that’s how he’s repaying it, with a little profit and a little margin for paying taxes for Michael.’”

Weisselberg would be one of the few people who could tell prosecutors definitively that the Cohen retainer was for “sham legal invoices with a nonexistent retainer agreement.” He could have corroborated Cohen’s claims about where the money came from, how it was paid and perhaps even Trump’s role in all of it.

That, in and of itself, is a big deal. After all, Cohen’s plea agreement makes clear that Trump is functionally an unindicted co-conspirator in an attempt to break campaign finance laws. To the extent Weisselberg played a role in that, his immunity has always mattered.

But, the bigger – and far scarier – question for Trump is whether Weisselberg’s grant of immunity is about more than just telling SDNY prosecutors what he knew about the Daniels and McDougal payments. As of right now, Weisselberg hasn’t been called back to testify, according to a source familiar with the matter who spoke to CNN.

If that changes, look out. The man who handles the money is always the one who knows the secrets. And, in a financial world as complex as the one Trump has constructed for himself over the past few decades, there is literally no one who knows more about the finances of the Trump Organization – and the man sitting in the Oval Office – than Allen Weisselberg.

It’s worth remembering just how little we know about Trump’s finances. He became the first major party presidential candidate in the post-Watergate era to refuse to release any of his past tax returns, insisting they were under audit and could not be released. (Richard Nixon released his tax returns while under audit in 1973.) As I’ve written many times before, that was a political decision by Trump; he decided that whatever was in the tax returns would create bigger problems than the heat he would take for not releasing them. So, if you are holding your breath for that IRS audit to end sometime soon and for Trump to release 10 years of his past returns, stop. Because you will die. You need to breathe.

Weisselberg is the key to understanding Trump’s finances. He may well be the only person who could possibly understand and explain the full scope of Trump’s financial life. And now, he has been granted immunity to talk about at least a small part of that world with prosecutors in New York.

That fact has to scare Trump. But, if Weisselberg’s immunity deal extends beyond what he knew about the Cohen and Daniels payments, look out. That would be the biggest domino – by far – to fall in the broadening investigation led by special counsel Robert Mueller.