"Mr. President, with all due respect, you are making a big mistake," Senate Minority Leader Chuck Schumer told reporters he told Trump when the President called him earlier.
Schumer added that it's time for a special prosecutor to lead an investigation and that "this investigation must be run as far away as possible from this White House."
Sen. Mark Warner, the top Democrat on the Senate intelligence committee, said Tuesday evening that he wanted the Senate panel to hear from Comey as part of its investigation into alleged Russian interference in the 2016 elections and questions about potential collusion between Russia and the Trump campaign.
"I still want him to come and testify, even as former FBI director," Warner said in an interview on CNN's "Erin Burnett OutFront."
Describing Trump's move as "outrageous," Warner issued a call to public servants and legislators to stand against overreach he compared to the Nixon era.
"What happened during the Nixon period, there were people of principle who stood up against some of then-President (Richard) Nixon's actions," Warner said. "I'm hoping in the coming days that we'll see either out of the administration, and frankly from a lot of my colleagues, a willingness to rise above partisanship."
Other prominent Democrats across Washington cited Nixon's behavior more than 40 years ago to describe Trump's decision to sack Comey following the recommendation of the attorney general and deputy attorney general.
"This is Nixonian," Pennsylvania Democratic Sen. Bob Casey said in a statement. "Deputy Attorney General Rod Rosenstein must immediately appoint a special counsel to continue the Trump/Russia investigation."
Casey cited Comey's testimony last week before Congress where he confirmed the Department of Justice's investigation of Russian meddling included "investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination."
"We are careening ever closer to a Constitutional crisis, and this development only underscores why we must appoint a special prosecutor to fully investigate any dealings the Trump campaign or administration had with Russia," said Sen. Ed Markey, a Massachusetts Democrat, in a statement.
House Democratic Leader Nancy Pelosi called for "an independent, bipartisan commission" following Comey's firing.
"The President's sudden and brazen firing of the FBI Director raises the ghosts of some of the worst Executive Branch abuses," Pelosi said in a statement. "We cannot stand by and watch a coverup of the possible collusion with a hostile foreign power to undermine American democracy."
At last week's hearing, Comey confirmed that the FBI's investigation into possible coordination between Trump's presidential campaign and Russian officials was continuing.