US President Donald Trump has denounced the appointment of a special counsel to investigate his campaign's ties with Russia, calling it an unprecedented "witch hunt" that "hurts our country terribly".
However, fellow Republicans expressed hopes the move would restore some calm to a capital plunged into chaos.
This is the single greatest witch hunt of a politician in American history!— Donald J. Trump (@realDonaldTrump) May 18, 2017
A day after appointing former FBI director Robert Mueller to lead the independent probe, deputy attorney general Rod Rosenstein appeared behind closed doors before the full Senate.
Politicians from both parties sought to question him about Mr Trump's firing last week of FBI director James Comey, which was followed by news that Mr Trump had shared secrets with the Russians and tried to stop Mr Comey from investigating former presidential adviser Michael Flynn.
"We'll get rid of the smoke and see where the actual issues lie," said Senator Tim Scott.
"I do think that the special prosecutor provides a sense of calm and confidence perhaps for the American people, which is incredibly important."
Mr Trump strongly disagreed, saying the appointment "hurts our country terribly".
During a briefing with news anchors, he said it "shows we're a divided, mixed-up, not unified country" and is "a very, very negative thing".
The justice department announced on Wednesday that Mr Mueller has been given sweeping powers to investigate Russian interference in the 2016 presidential campaign, including potential links between Moscow and Trump campaign associates.
Despite initially opposing the appointment of an independent counsel, House Speaker Paul Ryan said 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".
With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!— Donald J. Trump (@realDonaldTrump) May 18, 2017
But Mr Trump, after issuing a measured statement when the news first broke on Wednesday evening, allowed his resentment to burst forth on Thursday in angry morning tweets.
"This is the single greatest witch hunt of a politician in American history!" Mr Trump wrote, ignoring impeachment efforts and blistering verbal attacks on previous presidents and other political leaders.
"With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!" he added later, without providing examples.
The president's tweets and comments to the TV anchors drew little reaction from fellow Republicans, who instead joined Democrats in heaping praise on Mr Mueller.
He will have nearly unfettered access to witnesses and information, and the ability to bring criminal charges.
His appointment raises the stakes dramatically on the long-simmering allegations that Russia meddled in the 2016 election and had connections with members of the Trump campaign.
Democratic senators had been prepared to press Mr Rosenstein to take the step of appointing a special prosecutor, but were left praising him instead before his closed-door briefing began.
"This was a very good first step. Mr Rosenstein has done the right thing," Senate Minority Leader Chuck Schumer said on the Senate floor.
"I now have significantly greater confidence that the investigation will follow the facts wherever they lead."