New York: In the Trump era, the constant fog of controversy can make it hard to tell the difference between a game-changing crisis and a passing irritant.

But Tuesday's guilty plea by Trump's former fixer Michael Cohen was, without question, a disaster for a President who has so far been able to survive seemingly any scandal.

This was one of Trump's worst days since he came to power.

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The surest sign was his silence. Trump is known for his bellicosity and willingness to retaliate when under attack.  But when confronted by reporters about Cohen pleading guilty to eight charges, he avoided the topic, as he did at a rally on Tuesday night in West Virginia. Six hours after the plea deal became public, Trump's Twitter account still had nothing to say.

That's because Trump is now officially implicated in a criminal conspiracy to influence the 2016 election. And not by colluding with Russia to hack Hillary Clinton's emails, but rather by allegedly paying hush money to a porn star and a model to stop them blabbing about affairs with him.

When first revealed earlier this year, the Stormy Daniels story, centred on a $US130,000 payment to the adult film star, seemed a somewhat tawdry distraction from more important issues. It may well prove to be the defining scandal of Trump's presidency, and the question of Russian collusion, with all its apparent geopolitical gravitas, the sideshow.

That's because this touches Trump directly: it goes to his own behaviour, his truthfulness and the integrity of his marriage. Trump and his aides are far more spooked by the Cohen trial than by the conviction on Tuesday of his former campaign chair Paul Manafort (on charges unrelated to his time working for Trump).

On Tuesday Cohen told a New York court that “in coordination with and at the direction of a candidate for federal office,” he and the chief executive of a media company worked to keep an individual (former Playboy model Karen McDougal) from publicly disclosing information that could harm the candidate (Trump). Cohen said he worked “in coordination” with the candidate to make a payment to a second individual (Stormy Daniels).

“Today he stood up and testified under oath that Donald Trump directed him to commit a crime by making payments to two women for the principal purpose of influencing an election,” Cohen's lawyer Lanny Davis later said. “If those payments were a crime for Michael Cohen, then why wouldn’t they be a crime for Donald Trump?”

The reason is that, according to conventional legal wisdom, sitting American presidents are immune from routine criminal prosecution - a view long maintained by the Justice Department.

While Trump is unlikely to be charged while in office, if Cohen's claims are upheld they increase the likelihood of impeachment by the House of Representatives if the Democrats seize control in November.

The threshold for impeachment is "high crimes and misdemeanours" - a standard many Democrats may conclude is met by a presidential candidate violating campaign finance laws. Especially given the Trump campaign originally "unequivocally" denied any knowledge of the Daniels payment.

To paraphrase Martin Lawrence's iconic line from Bad Boys II: things just got real.