US President Donald Trump has slammed the FBI raid on his personal attorney Michael Cohen as a "total witch hunt" and a "disgrace".

He said the raid was distracting from other more "serious things" facing the country.

But what could be more serious for the US than the prospect of a criminal investigation, instigated by special counsel Robert Mueller, into a $US130,000 ($168,332) payment made days before the 2016 presidential election to silence a porn actress who allegedly had an affair with Mr Trump — a payment that arguably influenced the election result?

Whether Mr Trump likes it or not, this new probe — while ostensibly about the tax and business affairs of Mr Cohen — threatens to shine a legal spotlight on his own personal affairs, and any role his attorney played in keeping those affairs secret.

Trump's fix-it guy caught in the web

Mr Cohen has referred to himself as Mr Trump's "fix-it guy" — a role that has apparently included anything from having an engine repaired on Mr Trump's plane to representing his interests in a dispute with residents.

Some say there are dangers for Mr Trump in having Mr Cohen drawn into an FBI probe. ( Reuters: Stephanie Keith, file )

"If somebody does something Mr Trump doesn't like, I do everything in my power to resolve it to Mr Trump's benefit," he said in a 2011 interview.

"I'm going to come at you, grab you by the neck and I'm not going to let you go until I'm finished."

Mr Cohen's own attorney Stephen Ryan has acknowledged the raid — which seized records of communications between Mr Cohen and a range of clients — was mainly about the payment to actress Stormy Daniels, whose real name is Stephanie Clifford.

But some say there are dangers for Mr Trump in having his "fix-it guy" drawn into an FBI investigation.

Mr Trump has denied ever having an affair with Stormy Daniels. ( AP: Matt Sayles )

One Washington lawyer, Mark S Zaid, said the seizure of Mr Cohen's records in the raid "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 told the Washington Post.

"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."

Another attorney, Joyce White Vance, said the FBI search warrant was like "dropping a bomb on Trump's front porch".

So far, it's not clear what investigators are looking for in Mr Cohen's records.

But even to obtain a warrant, prosecutors need to convince a judge they have "probable cause of criminal activity" and they believe they'll find evidence of wrongdoing by conducting a search.

The storm that won't go away

Daniels is, of course, not the only woman who has alleged former affairs with Mr Trump — or even sexual misconduct on his part.

But she's the one who could eventually bring the President undone.

Mr Trump has denied ever having an affair with the actress — but on the other hand, his legal team is suing her for $US20 million ($25.8 million) in damages for breaking the non-disclosure deal.

Sorry, this video has expired The White House says Donald Trump denies all Stormy Daniels' claims

Many ask why such a deal was made if there was no affair to cover up.

But the more immediate question now for the FBI is whether the payment was illegal, and whether it breached election campaign finance laws.

If proved, that alone could amount to a criminal act and possible charges.

As the actress' own attorney Michael Avenatti stated last month, it would also constitute an abuse of power.

"This is about the cover-up," he told the US 60 Minutes program.

"This is about the extent that Mr Cohen and the President have gone to intimidate this woman, to silence her, to threaten her, and to put her under their thumb. It is thuggish behaviour from people in power."

Mr Cohen has always insisted he paid the actress out of his own pocket and that he was never reimbursed.

Last week Mr Trump claimed he didn't know about the payment.

Sorry, this video has expired Donald Trump denies knowledge of a payment to Stormy Daniels

But even that may be academic.

Government watchdog group Common Cause filed a complaint with the Federal Election Commission in January, alleging the payment was an "unreported in-kind contribution to President Trump's 2016 presidential campaign committee … and an unreported expenditure by the committee — because the funds were paid for the purpose of influencing the 2016 presidential general election".

The group alleged "regardless of the source of the funds" Mr Trump's campaign team failed to report its receipt of the contribution or its expenditure.

The complaint also alleged Mr Cohen created a new company, Essential Consultants LLC, less than two weeks before the election and "used a bank account linked to the entity" to pay the actress.

Importantly, the company was created in Delaware — where corporate transparency laws are reportedly more lax.

Will Trump fire Mueller?

The FBI raid shows, once again, Mr Mueller has shown he is prepared to act on information that may be well beyond the brief he was hired for, by referring matters to other authorities.

And it comes as Mr Trump and his legal team have been weighing whether he should agree to an interview with Mr Mueller, which would undoubtedly examine whether his actions amount to obstruction of justice.

Now, the President may be weighing whether to get in first, and sack Mr Mueller.

Then again, given Mr Mueller has already referred the Mr Cohen matter to the FBI, even that may be too late.