Susan Page

USA TODAY

Corrections & clarifications: An earlier version of this story misstated the length of time since President Nixon's resignation.

WASHINGTON — Not since Watergate.

FBI Director James Comey delivered more than one bombshell at a rare public hearing of the House Intelligence Committee Monday. He said there was no evidence to back up President Trump's accusation that President Obama wiretapped Trump Tower during the 2016 campaign, joining congressional leaders and intelligence officials in discounting Trump's unsubstantiated claims.

What was more explosive, though, was Comey's matter-of-fact confirmation that the FBI is investigating whether Trump associates colluded with Russia in the effort by one of the United States' leading global adversaries to affect the outcome of the 2016 presidential campaign.

To be sure, the FBI in recent years has been drawn into investigations involving the top ranks of the White House, from Scooter Libby's leaking of a CIA operative's name during the George W. Bush administration to President Bill Clinton's relationship with Monica Lewinsky. But not since the Watergate scandal that forced President Richard Nixon to resign nearly a half-century ago has there been an official investigation of such potential consequence.

The allegations “are more serious than anything we’ve seen in recent decades,” says political historian Matthew Dallek, a professor at George Washington University and author of Defenseless Under the Night: The Roosevelt Years and the Origins of Homeland Security. “That is far from a lone instance of an illegal leak or a president fudging the truth under oath about sex with a White House intern.” While we obviously don’t know what evidence the FBI has assembled and whether anybody will ever be charged with a crime, the scope of the investigation appears to encompass at least several of Trump’s chief advisers and perhaps Trump himself."

Read more:

FBI's Comey says Obama did not order wiretapping of Trump's New York office

Trump aide: 'Nothing has changed' in Russia hearing

Trump continues to defend wiretapping claims

The disclosure creates complications for the Trump Justice Department in overseeing the inquiry and is sure to fuel Democratic arguments that an independent counsel needs to be named. Though Comey declined to outline a timetable for the FBI inquiry, questions about whether some in the Trump team coordinated with Moscow’s meddling are all but certain to hang over the administration for months or even years.

Just ask veterans of the Iran-contra investigation in the Reagan White House or of the Monica Lewinsky investigation in the Clinton White House the sort of shadow they can cast.

Comey promised that the FBI would “follow the facts wherever they lead” — including whether crimes were committed. Because of extraordinary public interest, he said, the Justice Department had authorized him to take the unusual step of commenting on an active counterintelligence investigation. “If the Trump campaign, or anybody associated with it, aided or abetted the Russians, it would not only be a serious crime,” California Rep. Adam Schiff, the ranking Democrat on the panel, said in his opening statement. “It would also represent one of the most shocking betrayals of our democracy in history.”

After more than five hours of testimony, Intelligence Chairman Devin Nunes, R-Calif., closed the hearing with an almost anguished plea to Comey to finish the investigation as soon as possible. “The longer this hangs out here, the bigger the cloud is,” he said. “There is a big gray cloud that you have now put over people who have very important work to do to lead this country.”

The White House dismissed the whole idea as preposterous.

“James Clapper and others stated that there is no evidence Potus colluded with Russia,” Trump declared in the first

of a string of early-morning tweets Monday on his personal Twitter account, @realDonaldTrump. “This story is FAKE NEWS and everyone knows it!” He declared: “The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign.”

Then he tweeted: “The real story that Congress, the FBI and all others should be looking into is the leaking of Classified information. Must find leaker now!”

That was advice Nunes and other Republicans on the committee took, asking more questions about who might have leaked information to news organizations than they did about the substance of the information that was leaked. They focused in particular on the disclosure that a phone call by Trump adviser Michael Flynn was captured during surveillance of the Russian ambassador. The law calls for the identity of Americans to be masked when they are caught up in foreign eavesdropping.

“Some of that may rise to the level of a crime,” said Rep. Trey Gowdy, R-S.C.

There were signs that Trump was watching — and no signs that he was ready to temper his rhetoric.

While Comey was testifying, there was a tweet on the official @POTUS Twitter account. “FBI Director Comey refuses to deny he briefed President Obama on calls made by Michael Flynn to Russia,” it said — suggesting that the former president himself was behind the leak. Another tweet declared, “The NSA and FBI tell Congress that Russia did not influence electoral process.”

That wasn’t quite what they said, Comey and Rogers told the panel when the tweet was read to them, in what amounted to an awkward fact-checking exchange in real time with their boss. They said they hadn’t drawn conclusions on the impact of Moscow’s efforts.

Comey did predict that Russia’s efforts to disrupt American elections weren’t over.

“They’ll be back in 2020,” he warned. “They may be back in 2018. And one of the conclusions they may draw ... is they were successful.”