WASHINGTON — President Donald Trump abruptly fired FBI Director James Comey on Tuesday, throwing into chaos the politically fraught investigation into whether Russia had colluded with Trump’s campaign during the presidential campaign.

Democrats immediately renewed their calls for the appointment of an independent investigator in the Russia investigation.

“Any attempt to stop or undermine this FBI investigation would raise grave constitutional issues,” Sen. Dick Durbin, D-Illinois, said. “We await clarification by the White House as soon as possible as to whether this investigation will continue and whether it will have a credible leader so that we know it will have a just outcome.”

Sen. Angus King, I-Maine, a member of the Senate Intelligence Committee, said, “Based on what I know, it appears to me that the firing of the FBI director was a solution in search of a rationale. To dismiss Director Comey based on actions he took last year, when the president has been in office for more than five months, just doesn’t add up and raises more questions than it answers.

“Furthermore, his dismissal is especially troubling given that there is an ongoing FBI investigation into contacts between Russia and the Trump campaign. His firing undermines confidence in the credibility of that process, and to restore it – both for me and for the American people – I believe it is not only appropriate, but urgently necessary to appoint a special counsel to carry forward the investigation. Meanwhile, I expect the Senate Intelligence Committee investigation to continue to proceed in a nonpartisan manner and follow the facts wherever they lead.”

Sen. Susan Collins, R-Maine, said, “Today’s announcement is likely the inevitable conclusion of Director Comey’s decision last July to bypass the longstanding protocols of the Justice Department and publicly announce the reasons he had decided not to recommend an indictment of Hillary Clinton and to offer his personal views of Mrs. Clinton’s actions.

“That decision, while well-intentioned, embroiled Director Comey into political controversies that unfortunately continued to this day.

“Any suggestion that today’s announcement is somehow an effort to stop the FBI’s investigation of Russia’s attempt to influence the election last fall is misplaced. The president did not fire the entire FBI; he fired the director. I have every confidence that the FBI will continue to pursue its investigation. In addition, I am certain that the Senate Intelligence Committee, on which I serve, will continue its own bipartisan investigation and will follow the evidence wherever it leads.

“I hope that the next FBI director will have the same kind of integrity, intelligence, and determination that Mr. Comey exhibited, but perhaps better judgment on when it is appropriate to comment publicly on the results of an investigation.”

Comey was scheduled to appear before the Senate Intelligence Committee on Thursday. It was unclear whether that appearance would take place.

White House press secretary Sean Spicer told reporters at the White House that Comey was fired Tuesday afternoon after Trump received letters from Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein recommending the dismissal.

“While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau,” Trump wrote in a letter to Comey.

Spicer declined to answer additional questions, but said the firing was effective immediately. The FBI is being run by its deputy, Andrew McCabe, as the administration launches a search for a new leader.

Trump had asked Comey to stay on as FBI director even though he was overseeing the Russia investigation into collusion and the hacking and public release of top Democrats’ private emails.

Sen. Dianne Feinstein, D-California, a member of the Senate Intelligence Committee, said Trump called her at 5:30 p.m. to inform her that he was firing Comey because the FBI needed a change. She did not praise Comey, but in a statement merely said: “The next FBI director must be strong and independent and will receive a fair hearing in the Judiciary Committee,” she said in a statement.

Comey had a long career as a federal prosecutor, rising to deputy attorney general during the George W. Bush administration. Former President Barack Obama, a Democrat, appointed Comey to a 10-year term in 2013, but Trump had the prerogative of removing him from the job.

Comey, though never an FBI special agent himself, appeared to be well-regarded by many in the bureau. “Most of our members have a good deal of respect for him,” said Nancy Savage, executive director of the Society of Former Special Agents of the FBI.

“Given the recent controversies surrounding the director, I believe a fresh start will serve the FBI and the nation well,” said Sen. Lindsey Graham, R-South Carolina. “I encourage the president to select the most qualified professional available who will serve our nation’s interests.”

“This is Nixonian,” said Sen. Bob Casey, D-Pennsylvania. “Deputy Attorney General Rod Rosenstein must immediately appoint a special counsel to continue the Trump/Russia investigation. This investigation must be independent and thorough in order to uphold our nation’s system of justice.”

Rosenstein was charged with overseeing the Russia investigation after Sessions recused himself because of disclosures that he had met with Russia’s ambassador, while acting as a Trump campaign adviser.

Just two weeks after the Senate confirmed his nomination, Rosenstein signed a scathing memo Tuesday that assailed Comey’s unorthodox decision last July to publicly announce he would not recommend criminal charges against Democratic presidential candidate Hillary Clinton. Comey took the action just days after it was disclosed that Attorney General Loretta Lynch had met on the tarmac of an Arizona airport with Clinton’s husband, former President Bill Clinton.

Rosenstein accused Comey of usurping the attorney general’s authority on July 5, 2016, when he announced his conclusion that the case should be closed without prosecution. He blasted Comey for holding a press conference to “release derogatory information” about the subject of a declined criminal investigation. He accused Comey giving conclusive findings to the news media instead of presenting the bureau’s findings to federal prosecutors.

“It is a textbook example of what federal prosecutors and agents are taught not to do,” Rosenstein said.

Rosenstein appeared to pick comments of several former attorney generals, including Eric Holder, who served under Obama, and Alberto Gonzales, who served under Bush, to back his arguments.

“Almost everyone agrees that the director made serious mistakes; it is one of the few issues that unites people of diverse perspectives,” Rosenstein wrote.

Sessions, in a letter to Trump, said he also “concluded that a fresh start is needed at the leadership of the FBI.”

“It is essential that this Department of Justice clearly reaffirm its commitment to longstanding principles that ensure the integrity and fairness of federal investigations and prosecutions,” Sessions said. “The director of the FBI must be someone who follows faithfully the rules and principles of the Department of Justice and who sets the right example for our law enforcement officials and others in the department.”

Trump fired Comey despite praising him after he briefly reopened the investigation of Clinton just 11 days before last November’s election when a new cache of Clinton emails surfaced on the laptop computer of former New York Rep. Anthony Weiner, whose wife, Huma Abedin, was a top Clinton State Department aide. He said then that “it took a lot of guts” for Comey to reopen the inquiry.

Democrats have long called for an independent counselor or special prosecutor to look into the Russia inquiry.

Democratic Sen. Kamala Harris of California, seen as one of the future leaders of the party and a potential challenger to Trump, called for a special prosecutor.

“I’ve said it before and will again — we must have a special prosecutor to oversee the FBI’s Russia investigation,” she tweeted. “This cannot wait.”

Maine’s 1st District Rep. Chellie Pingree said, “Why is President Trump firing FBI Director Comey now? Why wasn’t he asked to resign on January 20th? Was the FBI making progress on the Russia investigation?

“For months I’ve called for an outside, special prosecutor who will give the American people the answers they deserve.”

Clinton’s former campaign spokesman Brian Fallon dismissed as nonsense the Trump administration’s claim that Trump lost confidence in Comey over his handling of the Democrat’s email investigation.

But some conservatives rallied to Trump’s defense. Judicial Watch President Tom Fitton picked up the White House talking points, saying the decision to fire Comey would restore the public’s trust in the FBI.

“Mr. Comey did not seem to understand some of the laws he was asked to investigate and unfortunately politicized his sensitive positon (sic) as the FBI director. President Trump took the right step in cleaning house at the FBI.”

Earlier Tuesday, the FBI wrote the Senate Judiciary Committee and said Comey recently overstated to Congress the number of emails Abedin forwarded to Weiner while working at the State Department. The FBI said just two of those messages contained classified information.

A search for a new permanent FBI director will begin immediately.