Democratic lawmakers demanded the appointment of a special prosecutor Tuesday night after FBI Director James Comey was fired.
"Now it's even more important that an independent special counsel is appointed to investigate the Russian interference in our elections," said New Jersey Senator Cory Booker.
"In case the need for an independent special prosecutor to investigate #TrumpRussia ties wasn't clear enough already…it sure is now," said Oregon Senator Jeff Merkley.
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New York Senator Charles Schumer used the word "cover up."
"If we don't get a special prosecutor, every American will rightfully suspect that the decision to fire #Comey was part of a cover-up," Schumer said.
President Donald Trump, who fired Comey, called top Judiciary Committee Democrat Sen. Dianne Feinstein before announcing the decision, her office said.
"The next FBI director must be strong and independent and will receive a fair hearing in the Judiciary Committee," her statement said.
Virginia Senator and former Hillary Clinton running mate Tim Kaine took to Twitter to call the firing of Comey a sign of "how frightened the Admin is over Russia investigation."
Some Democrats charged that the sudden ouster by the president of the FBI chief had echoes of Watergate.
"This is Nixonian," said Pennsylvania Senator Bob Casey. "Deputy AG Rod Rosenstein must
immediately appoint a special prosecutor to continue the Trump/Russia investigation."
New Mexico Senator Martin Heinrich said, "President Trump's dismissal of FBI Director Comey smacks of President Nixon's Saturday Night Massacre. … If this is an effort to stop the investigations into Russian collusion with the Trump campaign, it won't succeed."
Hawaii Senator Brian Schatz said the nation was in "a full-fledged constitutional crisis."
Vermont Sen. Patrick Leahy called the action and the White House's handling of it "shocking" and labeled it "nothing less than Nixonian."
"No one should accept President Trump's absurd justification that he is now concerned that FBI Director Comey treated Secretary Clinton unfairly," Leahy said. "The President has removed the sitting FBI director in the midst of one of the most critical national security investigations in the history of our country — one that implicates senior officials in the Trump campaign and administration."
Even Republican Senator Bob Corker of Tennessee said the timing of Comey's ouster "will raise questions."
"While the case for removal of Federal Bureau of Investigation Director James Comey laid out by Deputy Attorney General Rod J. Rosenstein was thorough, his removal at this particular time will raise questions," said Corker. "It is essential that ongoing investigations are fulsome and free of political interference until their completion, and it is imperative that President Trump nominate a well-respected and qualified individual to lead the bureau at this critical time."
North Carolina Senate Richard Burr, the chairman of the Senate Intelligence Committee, which is investigating Russian interference in the 2016 campaign, said he was "troubled" by the firing.
"I am troubled by the timing and reasoning of Director Comey's termination," Burr said in a statement. "Director Comey has been more forthcoming with information than any FBI Director I can recall in my tenure on the congressional intelligence committees. His dismissal, I believe, is a loss for the Bureau and the nation."
Burr added that Comey's "dismissal further confuses an already difficult investigation by the Committee."
Other Republican reaction was more muted.
"Given the recent controversies surrounding the director, I believe a fresh start will serve the FBI and the nation well," said South Carolina Senator Lindsay Graham.
Meanwhile, former Hillary Clinton campaign aides also were quick to weigh in.
"I'm very suspicious — there was certainly cause to do this — but not reasons President Trump has ever embraced before," Jennifer Palmieri told NBC News. "He certainly bungled the Clinton investigation, but I am suspicious for him to be fired while he was investigating the Trump campaign and its associates."
And ex-Clinton campaign manager Robby Mook called it the "twilight zone."
One Democratic staffer on the Hill told NBC News that while few on his side of the aisle are sad to see Comey go, the firing "feels like a Russian novel mixed with a spoof on Watergate."
The Nixon Library appeared to take issue with the comparison, tweeting, "FUN FACT: President Nixon never fired the Director of the FBI #FBIDirector #notNixonian."