Brimming with resentment, U.S. President Donald Trump fervently denied on Thursday that his campaign had collaborated with Russia or that he'd tried to kill an FBI probe of the issue, contending that "even my enemies" recognize his innocence and declaring himself the most unfairly hounded president in history.

Asked point-blank if he'd done anything that might merit prosecution or even impeachment, he said no and then added concerning the allegations and questions that have mounted as he nears the four-month mark of his presidency: "I think it's totally ridiculous. Everybody thinks so."

Trump denies allegations he interfered with FBI investigation into Michael Flynn 0:40

Not quite everybody. While Trump tweeted and voiced his indignation at the White House, Deputy Attorney General Rod Rosenstein — who appointed an independent special counsel to lead a heightened federal Trump-Russia investigation the day before — briefed the entire Senate behind closed doors at the Capitol.

Turmoil in Washington

By several senators' accounts, he contradicted Trump's statements that Rosenstein's written criticism of FBI Director James Comey had been a factor in Comey's recent firing by the president.

Trump is leaving Friday for his first foreign trip, to the Mideast and beyond, and aides had hoped the disarray at home would have been calmed if not resolved, allowing the White House to refocus and move ahead.

Republicans on Capitol Hill hoped the same, reasoning that the appointment of a special counsel could free them to work on a major tax overhaul and other matters without constant distractions.

Former Connecticut senator Joe Lieberman waves to members of the media as he leaves the West Wing of the White House on Wednesday. Lieberman is seen as a front-runner to lead the FBI. (Pablo Martinez Monsivais/Associated Press)

Trump said he was about to name a replacement for Comey, another move to settle the waters. Former Connecticut senator Joe Lieberman was seen as the front-runner.

But calmness seemed far off.

Trump clearly knew what he wanted to say as he took a few questions at a news briefing with visiting Colombian President Juan Manuel Santos.

Did he urge Comey at a February meeting to drop his probe of the Russia connections of Trump's national security adviser, Michael Flynn?

"No. No. Next question."

Did he in fact collude with Russia in his campaign to defeat Democrat Hillary Clinton?

"Everybody, even my enemies, have said there is no collusion," he maintained.

However, another answer on that subject seemed both more specific and perhaps ambiguous.

"There is no collusion between certainly myself and my campaign — but I can only speak for myself — and the Russians. Zero."

"The entire thing has been a witch hunt," he declared, echoing one of the tweets he'd sent out just after dawn: "This is the single greatest witch hunt of a politician in American history!"

This is the single greatest witch hunt of a politician in American history! —@realDonaldTrump

He said he respected the special counsel appointment, but also said it "hurts our country terribly."

At the other end of Pennsylvania Avenue, Rosenstein was briefing the Senate about his decision to appoint former FBI Director Robert Mueller to lead the independent Trump-Russia probe.

Senators said that Rosenstein steered clear of specifics while making clear that Mueller has wide latitude to pursue the investigation wherever it leads, including potentially criminal charges. Despite the president's furious reaction, some fellow Republicans welcomed Mueller's appointment and expressed hopes it would restore some composure to a capital plunged in chaos.

Former FBI director Robert Mueller, seen here testifying before a congressional committee in 2013, was on Wednesday named special counsel to investigate allegations of collusion between the Trump campaign and Russia. (Yuri Gripas/Reuters)

"We'll get rid of the smoke and see where the actual issues lie," said Senator Tim Scott, a South Carolina Republican. "I do think that the special prosecutor provides a sense of calm and confidence perhaps for the American people, which is incredibly important."

One striking piece of news emerged from Rosenstein's briefing: He told senators that he had already known Comey was getting fired even as he wrote the memo that Trump cited as a significant justification for the FBI director's dismissal. Trump himself had already contradicted that explanation, telling interviewers earlier that he had already decided to dismiss Comey.

He offered new justifications for his decision Thursday, even while referring to the Rosenstein memo as "a very, very strong recommendation.

Trump referred to Comey's testimony at a recent Capitol Hill hearing after which the Justice Department ended up having to amend part of his testimony regarding last year's probe of Hillary Clinton's email practices.

"That was a poor, poor performance," Trump said. "And then on top of that, after the Wednesday performance by Director Comey, you had a person come and have to readjust the record, which many people have never seen before, because there were misstatements made."

The Justice Department says Mueller, the new special counsel, has been given sweeping power to investigate Russian interference in the 2016 presidential campaign, including potential links between Moscow and Trump associates.

Despite initially opposing appointment of an independent counsel, Republican House Speaker Paul Ryan said Thursday that the development "helps assure people and the Justice Department that they're going to go do their jobs independently and thoroughly, which is what we've called for all along."

At the same time, congressional committees are continuing their own investigations, leading to some turf warfare and sniping as the Senate intelligence committee and the Senate judiciary committee both sought to lay claim to testimony from Comey, while the House oversight committee also hoped to hear from the former director.

The House intelligence committee announced that it had asked for documents from the FBI and the Justice Department.

The No. 2 Senate Republican, John Cornyn of Texas, said he was supportive of investigations in Congress, but expressed concern about the "proliferation" of hearings. "I hope that we don't inadvertently trip up or damage the independent investigation of the special counsel," he said.

The president's tweets drew little reaction from fellow Republicans, who instead joined Democrats in heaping praise on Mueller, a longtime respected lawman who served under George W. Bush and Barack Obama, preceding Comey as head of the FBI.

Committee chair resigns

Meanwhile, Republican Jason Chaffetz announced Thursday he will resign from Congress next month, saying a "mid-life crisis" compelled him to step away from his chairmanship of the House Oversight Committee as it is poised to launch an investigation into Comey's firing.

Chaffetz's announcement came a day after he tweeted that he had invited ex-FBI Director James Comey to testify next week at a hearing of the oversight committee he leads.

Chaffetz, a Utah Republican who had just started his fifth term in Congress, used his post as chair of the oversight committee to doggedly investigate Hillary Clinton before the 2016 presidential election and raise his political profile.

But he rescinded his endorsement of Trump last year after recordings surfaced of the reality show star bragging about groping women, only to hastily re-endorse Trump shortly before the FBI announced it was reviving its investigation into Clinton's emails.