The White House was rocked by two monumental developments on Tuesday.

Paul Manafort was convicted on eight counts related to financial fraud, and Michael Cohen pleaded guilty to tax evasion, bank fraud, and campaign finance violations.

Cohen said he broke the law "at the direction" of a 2016 presidential candidate widely believed to be President Donald Trump.

"This is, bar none, the worst day of the Trump presidency," said one legal expert.

Two massive legal developments Tuesday relating to President Donald Trump and his 2016 campaign threw Washington into a frenzy.

On Tuesday afternoon, Trump's former longtime personal lawyer, Michael Cohen, pleaded guilty to eight counts related to tax evasion, bank fraud, and campaign finance violations. Critically, he said he had made illegal campaign and corporate contributions "at the direction of" a 2016 presidential candidate and with the "purpose of influencing the election." That candidate is widely understood to be Trump.

Cohen's plea deal as of Tuesday does not include a cooperation agreement, and legal experts say that may be because prosecutors feel he does not have enough to offer them. However, they say that Cohen will likely continue trying to get a better deal by cooperating with any federal investigation — whether it's with the Manhattan US attorney's office, which was investigating him, or with another criminal investigation, like the Russia probe.

Elsewhere on Tuesday, a jury in Virginia convicted former Trump campaign chairman Paul Manafort of eight counts related to tax fraud, bank fraud, and failure to report foreign bank accounts.

Andrew Wright, a former associate in the White House counsel's office under President Barack Obama, didn't mince words when reacting to the events.

"This is, bar none, the worst day of the Trump presidency," he said.

'Hard to call it a witch hunt now'

Paul Manafort. Thomson Reuters

Trump and his allies in Congress and the right-wing media frequently deride the Russia probe as a politically motivated fishing expedition. While Trump has long attacked the investigation, his rhetoric heated up significantly this week, when he targeted the special counsel Robert Mueller himself in his most direct attack yet.

Following the Manafort verdict on Tuesday, Trump said, "It doesn't involve me, but it's a very sad thing that happened. This has nothing to with Russian collusion."

Trump added: "This started as Russian collusion. This has absolutely nothing to do — this is a witch hunt."

Mueller is tasked with investigating Russia's interference in the 2016 election. As part of that mandate, he is also looking into whether the Trump campaign colluded with Moscow, whether Trump sought to obstruct justice in the investigation, and any other matters that may arise out of the investigation.

Jens David Ohlin, a vice dean at Cornell Law School and an expert on criminal law, called the Manafort verdict "a big win for Mueller's team" that "certainly adds legitimacy to the Mueller investigation."

"Prosecutors found evidence of significant wrongdoing and a jury agreed with them," he said. "Hard to call it a witch hunt now."

Patrick Cotter, a longtime former federal prosecutor who has worked with members of Mueller's team, pointed out that the Russia probe and cases related to it "have resulted in over a half dozen persons being convicted of federal felonies."

"Everyone Mueller has targeted has pleaded guilty or been found guilty," he said. "There has not been one single 'not guilty.'"

Cotter added that Manafort's conviction also bolsters the special counsel's investigation by showing witnesses who may face criminal exposure "that failure to tell the truth and cooperate will lead to their being successfully prosecuted and sent to jail. This will mean more witnesses and more evidence for Mueller."

Mitchell Epner, a former federal prosecutor in the US attorney's office for the District of New Jersey, echoed that view and said "each conviction and guilty plea cuts another limb out from under [Trump's] continual cries of 'witch hunt.'"

He also pointed out that Mueller's continued progress will likely make it more difficult for Trump to garner congressional support to end the Russia investigation.

Manafort may choose to appeal his verdict. However, the former federal prosecutor Harry Sandick said that the judge's tough approach toward prosecutors and the jury's inability to reach a verdict on 10 out of the 18 counts against Manafort will likely make it difficult for the defense to prove his trial was unfair.

The biggest question for Manafort going forward: to flip or not to flip?

"Unlike other defendants — and this is where politics come into it — Manafort is uniquely positioned to know things about other facets of the Mueller probe that have to do with the Trump campaign, and that's why he's valuable," said Jeffrey Cramer, a former federal prosecutor in Chicago. "Would he talk about things other than what he's convicted for, about the president or anything else he knows?"

Cohen's guilty plea lands within striking distance of the White House

Michael Cohen. AP Photo/Mary Altaffer

Meanwhile, Cohen's guilty plea, though not connected to the Russia investigation, hits closer to home for Trump than any other development so far.

In particular, his statement Tuesday about violating campaign finance laws at Trump's direction opens the president up to significant liability, experts said.

Trump could face scrutiny "on some combination of solicitation, aiding and abetting, and conspiracy," Wright said.

Current Department of Justice (DOJ) policy states that a sitting president cannot be indicted. Legal scholars said that while Mueller or other federal prosecutors will likely not charge Trump because of that, he may be listed in court documents related to Cohen as an un-indicted co-conspirator.

Crucially, Epner noted that Trump would not be shielded from criminal prosecution once he leaves office.

Trump's lead defense lawyer, Rudy Giuliani, said Tuesday that "there is no allegation of any wrongdoing against the President in the government's charges against Mr. Cohen. It is clear that, as the prosecutor noted, Mr. Cohen's actions reflect a pattern of lies and dishonesty over a significant period of time."

But the former federal prosecutor Renato Mariotti pointed out that if Cohen's statements today were contradicted by other evidence prosecutors have, or if they did not believe Cohen, prosecutors would not have accepted the plea.

Cohen's guilty plea means he would be available as a witness in any potential prosecution of Trump during or after his presidency. He could also implicate others in criminal wrongdoing, who may in turn have evidence against Trump and testify to that in order to protect themselves.

"Either way, Cohen poses a serious threat to the president in a criminal law sense," Cotter said.

Wright agreed.

"If Donald Trump conspired with Cohen to commit felonies as a candidate, the only thing that might protect him is the question of whether he couldn't be indicted for the duration of his tenure in office," he said.