US President Donald Trump's Terrible Tuesday has made him an illegitimate president. There's growing evidence that Trump is, to use the label favoured by Richard Nixon, "a crook".

OPINION: Donald Trump's presidency is in danger of being undone by Terrible Tuesday - August 21, 2018.

Tuesday, (Wednesday, NZ time) was the day when the US President's campaign manager, Paul Manafort, was convicted of eight felony counts in a Virginia courtroom and his personal lawyer, Michael Cohen, pleaded guilty to eight felony counts in a New York courtroom.

The Manafort charges of tax and bank fraud do not directly implicate the president, but they do vindicate the special counsel, Robert Mueller, showing that his inquiry is no "Rigged Witch Hunt" but a serious investigation that has produced 35 indictments, six guilty pleas and one conviction.

No special counsel has done more, faster. If Manafort had been found not guilty, it would have been a massive blow to Mueller. Because he was found guilty, it is a blow to Trump.

READ MORE:

* Michael Cohen's Hillary Clinton tweet which continues to haunt him now gone

* Donald Trump calls Manafort a 'brave man', says he refused to 'break' unlike Cohen

* Cohen's claim may spark Donald Trump impeachment calls, but charges would be unlikely

* Michael Cohen pleads guilty to campaign finance charges, says Trump directed hush money

* Hannity had a lot to say about Cohen lately; but he left a few things out

* Trump-Russia probe: Robert Mueller's team has prosecuted high-stakes cases, including obstruction of justice

But not nearly as big a blow as Cohen's admission under oath that he violated federal campaign laws by arranging illicit payments to adult-film star Stormy Daniels and Playboy playmate Karen McDougal "in co-ordination with and at the direction of a candidate for federal office". For the first time since Watergate, the president is now an alleged co-conspirator in the commission of a federal crime.

ALEXANDREIA DETENTION CENTER VIA AP Paul Manafort, the longtime political operative who for months led Donald Trump's winning presidential campaign, was found guilty of eight financial crimes in the first trial victory of the special counsel investigation into the president's associates.

As Cohen's lawyer, Lanny Davis, said, his client "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. If those payments were a crime for Cohen, then why wouldn't they be a crime for Donald Trump?"

And Cohen may have only begun implicating the president.

Davis said on MSNBC that Cohen would be happy to share other incriminating information with the special counsel, including "knowledge about the computer crime of hacking and whether or not Mr Trump knew ahead of time about that crime and even cheered it on". This would seem to vindicate an earlier leak that Cohen may be able to provide the "smoking gun" evidence showing that Trump himself gave the go-ahead to collusion with the Kremlin.

ELIZABETH WILLIAMS/AP In this court room drawing, Michael Cohen swears that his statements will be truthful during a hearing at federal court in New York, on Tuesday, August 21. Cohen, the personal lawyer and "fixer" who once said he would "take a bullet" for Donald Trump, struck a deal with prosecutors to plead guilty to charges including campaign finance fraud, bank fraud and tax evasion.

In short, there is growing evidence that the president is, to use the label favoured by Richard Nixon, "a crook". Even buying the silence of his reputed playmates could by itself have been enough to swing an exceedingly close election decided by fewer than 80,000 votes in three states.

Trump certainly would not have authorised the payments unless he thought it was politically imperative to do so.

There is also considerable evidence, as I previously argued, that Russia's intervention on Trump's behalf affected the outcome. Even more than Nixon, Trump is now an illegitimate president whose election is tainted by fraud.

CRAIG RUTTLE/AP Michael Cohen told the court he violated federal laws "in co-ordination with and at the direction of a candidate for federal office". He promises to give prosecutors even more details.

The inevitable question is: Now what? If Trump had an iota of decency, he would resign. But he doesn't, and prevailing Justice Department guidelines hold that a president can't be indicted while in office. So the onus is on Congress to act.

A responsible Congress would have already convened an impeachment inquiry by now. But that is not the Congress we have. We have a Congress dominated by political hacks and moral invertebrates who are determined to act as the president's enablers and legitimisers at all costs.

It tells you all you need to know about the moral standing of Trump's defenders that Terrible Tuesday also included the indictment of one of his earliest congressional supporters, Representative Duncan D Hunter, Republican-California, and his wife on charges of misusing campaign funds to pay personal expenses. This comes shortly after another of Trump's early endorsers, Representative Chris Collins, Republican-New York, decided not to seek re-election after being indicted on insider-trading charges.

President Donald Trump repeated his claim that the Russia investigation is a "witch hunt" at a rally in Virginia.

But the far more serious crimes of Trump's congressional supporters do not involve personal peculation. They involve violating their oaths of office by failing to hold the president accountable for misusing his office.

Some, such as Representative Devin Nunes, Republican-California, and Representative Jim Jordan, Republican-Ohio, have gone much further by actively attempting to impede the Justice Department investigation into Trump's alleged misconduct. They have become, in a moral if not legal sense, accessories to obstruction of justice.

And they have got away with it because the congressional leadership has allowed them to do so. Judging by Tuesday's cowardly and cautious statement from House Speaker Paul Ryan, Republican-Wisconsin: ("We are aware of Mr Cohen's guilty plea to these serious charges. We will need more information than is currently available at this point,") there is no sign that the Republicans in Congress will ever provide any serious oversight of the Republican in the White House.

The voters of the United States must now say to this Congress what Oliver Cromwell said to the Rump Parliament in 1653: "Ye are a factious crew, and enemies to all good government. ... Ye are grown intolerably odious to the whole nation. You were deputed here by the people to get grievances redressed, are yourselves become the greatest grievance. ... Go, get you out! Make haste! ... In the name of God, go!"