"The President has lost confidence in Director Comey and, frankly, he'd been considering letting Director Comey go since the day he was elected," Sanders told reporters, adding that Deputy Attorney General Rod Rosenstein had raised concerns about the former FBI chief on Monday, prompting Trump to ask him to put his recommendation in writing.
Revelations that Trump had been unconvinced by Comey for his entire transition and presidency were surprising given the White House had previously expressed confidence in the ousted FBI chief multiple times. Sanders also said Trump was not the only one to lose confidence in the FBI director, adding there were deep reservations about him inside the FBI.
Sanders also said that Trump didn't regret not firing Comey sooner, saying he "wanted to give Director Comey a chance."
She also pointed to a "big catalyst" from last week congressional testimony by the former FBI chief when he related how he held a news conference without notifying his superiors last year. In the blockbuster appearance, Comey announced that Hillary Clinton would not face charges over her email server but also heavily criticized her handling of classified material.
"Director Comey made a pretty startling revelation that he had essentially taken a stick of dynamite and thrown it into the Department of Justice by going around the chain of command," Sanders said.
"That is simply not allowed," she said.
Sanders also characterized Comey's actions surrounding that appearance as "atrocities."
"I think also having a letter like the one that he received (from Rosenstein) and having that conversation that outline the basic atrocities in circumventing the chain of command in the Department of Justice" impacted Trump's decision, Sanders said.
Comey asked for more resources
It was revealed Wednesday that Comey asked the Justice Department for more resources for the agency's investigation into Russian meddling of the US election and ties to Trump's presidential campaign, just the week before the President fired him.
Comey told the heads of the Senate intelligence committee that he went to Rosenstein last week and pushed for more resources to be devoted to the Russia investigation, according to two sources familiar with the discussion.
Justice Department spokesperson Sarah Flores denies Comey asked Rosenstein for more resources. She called reporting to the contrary "100% false," and told reporters she spoke directly with Rosenstein about the reports.
Senate Intelligence Chairman Richard Burr, R-North Carolina, and Democratic Sen. Mark Warner of Virginia met Monday with Comey and urged him to speed up the Russia investigation. Comey acknowledged in this discussion that things were moving slowly.
Warner told CNN Wednesday he has asked Comey to testify before the committee Tuesday. It is an invitation only at this point -- no word yet if he will appear.
Before Rosenstein, Comey had been working with the acting deputy attorney general Dana Boente. Comey's requests for funds was first reported by The New York Times
In the wake of Comey's firing, a fierce political battle has erupted, as Democrats step up demands for a special prosecutor but Republican leaders hold the line to shield the White House.
With Washington still in disbelief over Tuesday's dramatic events that sent shockwaves across the nation, Trump blasted criticism of his sudden move, declaring that the former FBI chief "wasn't doing a good job."
But the President's comments did little to dispel the controversy over the firing of Comey: the FBI director overseeing the bureau's probe into whether there were any links between Trump campaign aides and Russia at a time when Moscow is accused of meddling in last year's election.
Critics claimed that Trump was trying to shut down the FBI Russia investigation. They ridiculed the White House's rationale for the firing -- that it was over Comey's handling of the Clinton email investigation that many Democrats believe cost the former secretary of state the presidency.
Trump met with with Acting FBI Director Andrew McCabe Wednesday afternoon, a White House official said.
The substantive push back was left to Vice President Mike Pence, who praised his boss for showing strong leadership to restore public confidence in the FBI and refuted suggestions the administration had anything to hide over Russia.
"President Trump made the right decision at the right time to accept the recommendation of the deputy attorney general and the attorney general, to ask for the termination, to support the termination of the director of the FBI," Pence told reporters on Capitol Hill.
The vice president also reiterated the White House's line that former Director of National Intelligence James Clapper has said that there is no evidence to support the idea of collusion between Trump aides and Russia.
But he would not answer a question about whether the impetus for the report by Rosenstein
that led to Comey's dismissal came from the White House or the Justice Department.
Pence's appearance before the cameras was a strategic decision of the White House to push back on the political crisis that ensued after Trump's move. The next White House official to face the public will be deputy press secretary Sarah Huckabee Sanders, who will brief reporters at 1:30 p.m. ET.
Trump joined the counter attack on Twitter as he lambasted his critics and Democrats who are accusing him of an abuse of power.
"Comey lost the confidence of almost everyone in Washington, Republican and Democrat alike. When things calm down, they will be thanking me!" Trump tweeted.
"The Democrats have said some of the worst things about James Comey, including the fact that he should be fired, but now they play so sad!" Trump wrote, and said a new FBI director would do a far better job.
But his comments did nothing to quell the sense of shock and uncertainty over what comes next.
Congress' next steps
In a highly unusual move, large numbers of Democrats showed up for the opening of Senate business on Wednesday, as Minority Leader Chuck Schumer called for a closed briefing of all senators with Comey and renewed his calls for a special prosecutor.
"Nothing less is at stake than the American people's faith in our criminal justice system and the integrity of the executive branch of our government," Schumer said.
But Senate Republican Majority Leader Mitch McConnell sought to quell the rising pressure on the White House.
McConnell argued that Democrats were "complaining" about Comey's removal after having argued during the campaign that Comey had behaved unfairly toward Clinton.
"Partisan calls," he said, shouldn't slow down the investigation by the Senate intelligence committee.
However, some other Republican senators are expressing increasing concerns about the escalating political crisis and the firing of Comey.
Arizona Sen. John McCain renewed his calls for a special congressional committee to investigate allegations of Russian election meddling.
"When you fire probably, arguably, the most respected person in America you better have a very good explanation and so far I haven't seen that," McCain told CNN's Manu Raju.
McCain said the White House rationale that Comey was fired because of his handling over the investigation into Clinton's emails was not sufficient.
Trump's demeanor suggested defiance on Wednesday but also verged on the surreal.
With questions swirling about his links with Russia, he met with Russian Foreign Minister Sergey Lavrov in the Oval Office for talks he later said were dominated by Syria.
The Russian embassy, meanwhile, tweeted a picture of Trump shaking hands with Ambassador Sergey Kislyak, who is at the center of the Russia storm since his conversations with Michael Flynn led to the firing of the former national security adviser.
Then Trump sat down with Henry Kissinger, who served as secretary of state under President Richard Nixon, apparently unconcerned that the photo-op could fuel already circulating allusions to the Watergate scandal in the 1970s.
Developments and revelations about the Comey firing tumbled out at a frenetic pace on Wednesday.
Longtime Trump ally Roger Stone was among those who recommended to the President that he fire Comey, according to a source familiar with the conversation. Stone, who also is being scrutinized by the FBI as part of its Russia investigation, spoke to Trump after Comey appeared at a hearing last week on Capitol Hill, according to the source.
Trump immediately disputed that Stone was involved: "The Roger Stone report on @CNN is false - Fake News. Have not spoken to Roger in a long time - had nothing to do with my decision."
Stone said he was not the source behind the reporting but added that he supported the firing of Comey "100%."
A White House spokesperson reached out to CNN to reiterate the President's tweet that he has not spoken to Stone in a "long time."
"The President has not spoken to Roger Stone in many months and any reports suggesting otherwise are categorically untrue," the spokesperson said.
Comey's dismissal ends a major chapter in a career of the man once seen as the unimpeachable and nonpartisan ideal of how a law enforcement officer should behave. But Comey saw his reputation tarnished when he was dragged into the toxic politics of the 2016 campaign.
In a signed letter released by the White House, Trump informed Comey that he was "hereby terminated and removed from office, effective immediately," explaining that he reached the conclusion that Comey is "not able to effectively lead the bureau."
White House press secretary Sean Spicer, sandwiched in the dark Tuesday night between a gaggle of reporters and a large shrub on the White House driveway, described Trump's decision as arriving only after a long memo from Rosenstein
, which Spicer said was delivered Tuesday, detailing Comey's shortcomings.
But multiple White House officials said Trump had been considering firing Comey for at least a week before he made Tuesday's decision. Indeed, Trump revealed his anger in a string of late-night messages on Twitter May 2, exactly a week before his final decision was made public.
Trump's decision came on a day when CNN reported that federal prosecutors had issued grand jury subpoenas to associates of former national security adviser Michael Flynn seeking business records, as part of the ongoing probe of Russian meddling in last year's election, according to people familiar with the matter.
The top senators on the Senate intelligence committee, Republican Richard Burr and Democrat Mark Warner, met Comey on Monday and urged him to speed up the Russia investigation, CNN's Sara Murray reported.
The search for Comey's successor is meanwhile already underway.
Attorney General Jeff Sessions and Rosenstein are interviewing potential interim FBI directors Wednesday, according to a Justice Department official. That same source said Andrew G. McCabe, who became acting FBI director yesterday, is on the list of interviewees.