Robert Reich said that it is no longer a question of if, but when the process to remove the president from office begins.
In the aftermath of last week’s firing of Mr Comey, the White House scrambled to provide an explanation for Mr Trump’s actions.
Officials sought to protect the President from accusations by Democrats and others, that he had fired him because he was heading the investigation into the possible collusion between the Trump campaign and Russia’s alleged efforts to influence the presidential election.
By my count, there are now four grounds to impeach Trump. The real question now is whether there's the political will. pic.twitter.com/OmgFygAcTP
— Robert Reich (@RBReich)
Writing for the Alternet news magazine, Professor Reich said Mr Trump’s own statements on why he fired Mr Comey provided “ample” evidence that he engaged in “the obstruction of justice” - a charge which led to impeachment proceedings being brought against both Presidents Clinton and Richard Nixon.
“The question is no longer whether there are grounds to impeach Donald Trump," said the academic and author, who was US Secretary of Labour under President Bill Clinton from 1993 to 1997 and previously served in the administrations of Presidents Gerald Ford and Jimmy Carter.
"It is when enough Americans will put their loyalty to America ahead of their loyalty to the party.”
Nixon was only President apart from Trump to fire his Attorney General
He added: “It’s worth recalling that the illegality underlying Nixon’s impeachment was a burglary at the Watergate complex, while the illegality underlying Clinton’s was lying to a grand jury about sex with an intern in the White House.
“Trump’s obstruction is potentially far more serious. It involves an investigation about whether Trump or his aides colluded with Russia in rigging a presidential election – the most direct assault on American democracy in history.”
Mr Trump insisted last week, that he was going to fire Mr Comey “regardless of recommendation”. But he also admitted that he pressed Mr Comey during a private dinner to tell him whether he was personally under investigation.
The US leader later took took to Twitter to say that Mr Comey “better hope that there are no ‘tapes’ of our conversation before he starts leaking to the press!”
Mr Reich said that the barely concealed threat from the President also amounts to an impeachable offence.
“Here, the law is also clear. Seeking to silence, intimidate or even influence someone who is likely to offer evidence in a congressional or criminal proceeding is also an obstruction of justice – and an impeachable offence,” he wrote.
The decision on whether to impeach Mr Trump would ultimately be decided by the US House of Representatives.
Under the current Congress, 22 members of Mr Trump's Republican Party would have to join with House Democrats to apply pressure on Speaker of the House, Republican Paul Ryan, to allow such a bill to be heard.
Donald Trump is going to be impeached soon, according to five experts
Mr Ryan described Mr Comey as “a dedicated and worthwhile public servant” last week, but deflected questions when he was asked about the bombshell firing.
“I'm going leave it to the President to talk about and defend his tweets,“ Mr Ryan said.
Professor Reich conceded that under the usual run of things, somebody would have to find a “smoking gun” on Mr Trump for Republicans to move to impeach their own party’s President.
As a result his fate could hinge on the midterm elections of 2018, when the balance of the House could change.
But the commentator predicted that if Mr Trump’s approval ratings in the polls continue to plummet, particularly among independent and Republican voters, it was not inconceivable that 22 Republicans could decide to move against their President rather than risk losing their own jobs and control of the House in the midterms, which are just 18 months away.
If Mr Trump was forced to resign, as President Nixon was, Mr Reich claimed that “most House Republicans prefer Vice President Mike Pence to Donald Trump anyway”.
The last straw for Mr Trump could be if the economy goes into recession, he wrote.
“He’ll be fired when enough Americans decide they can’t abide him anymore… it will come out that Trump did something incredibly stupid – like giving a nod of approval to one of his campaign bottom feeders like Roger Stone to tell a Russian operative to go ahead with their plan to interfere in the 2016 election.”
Acting head of the FBI Andrew McCabe has said the bureau will continue its probe into the Russia affair - but will not routinely update the White House on the investigation after calls for a fully independent enquiry.