Cohen is Trump's virtual vault - the keeper of his secrets, from his business deals to his personal affairs - and the executor of his wishes.
"This search warrant is like dropping a bomb on Trump's front porch," said Joyce White Vance, a former US Attorney in Alabama.
Mark Zaid, a Washington lawyer, said the seizure of Cohen's records "should be the most concerning for the President."
"You can't get much worse than this, other than arresting someone's wife or putting pressure on a family member," he said.
"This strikes at the inner sanctum: your lawyer, your CPA, your barber, your therapist, your bartender. All the people who would know the worst about you."
The President spent much of today glued to the television. Aides said Trump watched cable news coverage of surprise raids on Cohen's Manhattan office, home and hotel room by FBI agents, who took the lawyer's computer, phone and personal financial records after a referral from Mueller.
As the sun began to fall in Washington, Trump offered reporters his initial reaction: "It's a disgraceful situation."
"I have this witch hunt constantly going on," Trump said. "That is a whole new level of unfairness," he added, leaving no doubt that he views today's actions as a personal affront.
Trump called Cohen "a good man" and went on to criticise Attorney-General Jeff Sessions, saying he had made "a very terrible mistake for the country" by recusing himself from the Russia probe.
Asked why he had not fired Mueller, Trump left the door open.
"We'll see what happens," he told reporters. "Many people have said, 'You should fire him,' " the President added.
Shortly after the raids began, Trump received a heads-up at the White House. He huddled in the Oval Office with Ty Cobb, the White House lawyer who oversees its handling of the Mueller probe, as well as with White House counsel Donald McGahn and White House Chief of Staff John Kelly, officials said.
Other aides said they did not understand what was happening and struggled to pinpoint the significance of the seizures. Many officials sought to keep their distance from the developments, deferring comment until a strategy was determined.
Aides said they viewed Trump's comments to reporters as a necessary venting session. He had been grousing privately about Deputy Attorney-General Rod Rosenstein, a Trump appointee who oversees the Mueller investigation because of Sessions' recusal.
He complained about Rosenstein again in private, a White House adviser said, and stewed all afternoon about the warrant to seize Cohen's records, at times raising his voice. Trump said that Rosenstein approved the warrant, that he wished Rosenstein was not in the job and there was no one making the prosecutors follow the rules, the adviser said. Trump complained sharply about Sessions and Mueller and asked detailed questions about who was behind the move - and said that people would be more critical of such a warrant if it wasn't intended to damage the President.
Still, a senior White House official said that no "imminent" personnel changes were expected.
It was unclear if Trump talked to Cohen, with whom he recently dined at Mar-a-Lago, the president's private club in Palm Beach, Florida.
Trump "won't like that Cohen is in the crosshairs, but you have to remember: He'd prefer the heat be on Cohen than on him," said one of the President's advisers. "His goal will be to figure out how much vulnerability he has."
This was Trump's first crisis without Hope Hicks, the recently departed White House communications director who knew her way around the broader Trump orbit, getting to the bottom of what was happening, counselling the President and intuiting how he would want the situation handled.
Trump also navigated today's turn without a full slate of legal advisers. He has yet to replace John Dowd, who resigned last month as his personal lawyer in the Russia matter.
One White House official sighed when asked about Trump's strategy, pointing to the "evident" limitations of the current legal team, as well as the absence so far of a public-relations plan to counter the hotly anticipated release next week of former FBI Director James Comey's memoir, A Higher Loyalty.
There was fear in Trump's orbit that the President is liable to erupt in anger in coming days, escalating his attacks against Mueller at a time when his lawyers are negotiating a possible interview. And there was concern in some quarters that Trump, who has been shaking up his Administration in recent weeks, may also seek to terminate Mueller.
Former New York Mayor Rudy Giuliani, a friend of Trump, called the Cohen raids "a little heavy-handed."
"Is this surprising? Yes," said Giuliani, also a former US Attorney. "Is it extraordinary? No. This is the way prosecutors get information - sometimes to convict and prosecute, sometimes to exculpate."
Criticising Mueller for veering into "highly personal issues," such as the alleged Daniels encounter, Giuliani added, "The only thing that's happening, perhaps, is that Mueller is trying to compel the President to testify."
Trump told reporters aboard Air Force One last week that he did not know Cohen had arranged the US$130,000 payment for Daniels, whose real name is Stephanie Clifford, just days before the 2016 election to prevent her from publicly speaking about her alleged 2006 sexual encounter with Trump.
The President said he did not know where Cohen got the money and declined to answer whether he had set up a fund for Cohen to use. "You'll have to ask Michael Cohen," Trump said. "Michael's my attorney, and you'll have to ask Michael."
White House press secretary Sarah Huckabee Sanders has said repeatedly that Trump denies Clifford's allegations.
Without a lead lawyer in Dowd's absence, Trump has absorbed some advice from a number of legal commentators on cable news, including Alan Dershowitz, a retired Harvard Law School professor who has made supportive comments about the President.
"This may mark the end of the kind of cooperation that Trump's lawyers have been involved with," Dershowitz said today in an interview. "Cooperation doesn't seem to have much payback. Maybe it's better to go into a defensive fight mode."
Dershowitz advised Trump to use "every legal tactic available to him" to fight Mueller and the FBI. He said the President could "assert" his rights as Cohen's client and "go into court and seek to demand returned every bit of information that is arguably lawyer-client privilege before anybody has a chance to read anything."
Tim O'Brien, author of the Trump biography Trump Nation, said the seizure of records from his private lawyer probably would "smell of a mortal threat" by Trump. And, O'Brien added, "He is historically prone not to sit back and let the chips fall where they may. He is historically prone to come out with guns blazing."
Cohen has long been a fixer for Trump, as well as his family and business, and associates said he was disappointed when he was not brought officially on board the campaign, and again when he was passed over for a coveted White House job.
"He's done the dirty work that the President hasn't wanted to do himself, and he's been doing it for a decade," O'Brien said.
In the early weeks of the Administration, Cohen was spotted unshaven, roaming the lobby of the Trump International Hotel in Washington. He has stayed in touch with the President through late-night phone calls.
But now, Cohen is back squarely in Trump's orbit - though perhaps not in the way he had hoped to be. Cohen himself has become the kind of distraction that he was usually tasked with handling for his boss.
"When it comes to Michael Cohen, anything is possible," said Louise Sunshine, a former Trump Organisation executive who knows Cohen. "Anything and everything is possible."
