Judge Andrew Napolitano said prosecutors had made clear "that the president of the United States committed a felony by ordering and paying Michael Cohen to break the law".
“The felony is paying Michael Cohen to commit a felony,” Napolitano told Fox host Shepard Smith. “It’s pretty basic.”
Trump is unlikely to face indictment when in office because of Justice Department guidelines that presidents are immune from criminal prosecution. But he could potentially be charged when he leaves office or could be forced out of power if both houses of Congress impeach him for high crimes and misdemeanours.
In an opinion piece for The Washington Post, high-profile lawyer George Conway, the husband of Trump adviser Kellyanne Conway, wrote: "Trump could become a target of a very serious criminal campaign finance investigation.
"[T]he campaign finance violations here are among the most important ever in the history of this nation — given the razor-thin win by Trump and the timing of the crimes, they very well may have swung a presidential election," he wrote.
The piece was co-authored by Republican lawyer Trevor Potter and Neal Katyal, former acting attorney general in the Obama administration.
Trump's lawyer Rudy Giuliani dismissed the payments as "much ado about nothing".
“Nobody got killed, nobody got robbed," he told The Daily Beast. "This was not a big crime."
In an interview on ABC News on Friday local time Cohen said that Trump directed him to make the payments to porn star Stormy Daniels and Playboy model Karen McDougal.
“Nothing at the Trump Organisation was ever done unless it was run through Mr Trump,” Cohen said. “He directed me to become involved in these matters.”
Cohen said he would be happy to appear before a congressional committee to answer questions about the payments before he goes to jail next year.
The Wall Street Journal revealed on Thursday local time that prosecutors in New York were investigating whether Trump's inaugural committee misspent some of the record US$107 million it raised from donations.
Prosecutors are also reportedly investigating whether foreign donors illegally gave money to the committee and whether donations were given in exchange for political favours.
Trump's inaugural committee raised twice what Barack Obama's did in 2009, and the huge amount has long raised eyebrows.
Among the big donors to Trump's inauguration was Claudine Revere, the wife of Australian packaging mogul and Trump supporter Anthony Pratt.
Revere, a New York caterer and event planner, gave US$1 million to the inauguration fund. The Sydney Morning Herald and The Age do not allege any wrongdoing.
Former US attorney Chuck Rosenberg said: "The Trump Organisation more and more looks to be a criminal organisation".
He said that the inquiry into his inauguration suggested "a possible public corruption, pay-to-play bribery case in which incoming officials or officials sold access".
"That’s illegal," he told MSNBC.
Matthew Knott is a Fairfax Media reporter based in the United States. He previously worked in the Canberra press gallery and recently finished a Masters of Journalism at Columbia University in New York.