Washington (CNN) FBI Director James Comey provided Hillary Clinton a reprieve less than 48 hours before Election Day -- but the damage he inflicted on her campaign may already be done.

Comey's initial letter to Congress on October 28 was the very definition of the October surprise, rocking Clinton's campaign and giving Donald Trump's team a surge of momentum at a time when Clinton seemed to be coasting toward victory.

Eleven days before the election, Comey announced the FBI had discovered new emails that could be pertinent to the investigation into the former secretary of state's private email server, an inquiry that he had said was concluded this summer.

On Sunday, two days before the election, Comey essentially lifted that cloud of suspicion from her campaign -- signaling that his agency's new discovery of emails would not lead to criminal charges against Clinton.

"Based on our review, we have not changed our conclusions that we expressed in July," Comey said in his letter to top congressional Republicans.

Millions of votes already cast

While the news was welcomed by Democrats who worried that the initial announcement had inflicted serious damage to her campaign, it came after a week in which millions of early votes were cast around the United States. Moreover, many voters made up their minds in the last week, leaving Democrats furious about the last-minute announcement.

Though the race between Clinton and Trump was always expected to tighten in the final days, Comey's initial announcement was expected, at the very least, to depress turnout on Clinton's behalf. Her allies and advisers worry it has cost her votes among independents and Republican women who -- repelled by Trump -- had considered supporting her campaign.

A senior Democrat close to the Clinton campaign told CNN's Jeff Zeleny Sunday that it was "impossible to fully undo the damage of the last nine days," but acknowledged there was a sense of relief within the campaign that the matter was resolved.

California Sen. Dianne Feinstein, a ranking member of the Senate Judiciary Committee and the top Democrat on the Select Committee on Intelligence, said Sunday that Comey's initial letter "unfairly hurt the campaign of one candidate and changed the tenor of this election."

"Today's letter makes Director Comey's actions nine days ago even more troubling. There's no doubt that it created a false impression about the nature of the agency's inquiry," Feinstein said in a statement. "I believe the Justice Department needs to take a look at its procedures to prevent similar actions that could influence future elections."

Dan Pfeiffer, a former senior adviser to President Barack Obama, said he believed the damage to Clinton's campaign had been overstated.

"Much of what you saw was natural tightening, returning to the norm of what the race is, which is a three- to four-point race," he said.

"What this gives (the Clinton campaign) is a sense of momentum going forward," he said, noting that it also deprives Trump of a key talking point.

'Good news for Clinton'

"There's no question it's good news for Clinton," Pfeiffer said. "I don't think the original letter was going to cost her the campaign and I don't think this is going to win it for her. But you want to feel like the wind is at your back going into the election, and this certainly helps."

Republicans, including Trump, had roundly praised Comey for his surprising disclosure of the new email review more than a week ago. But the FBI director was quickly relegated to a GOP punching bag Sunday -- as Trump continued to argue on the campaign trail that "the system" was rigged to protect the powerful.

"Hillary Clinton is guilty," Trump said in Sterling Heights, Michigan. "She knows it. The FBI knows it. The people know it, and now it's up to the American people to deliver justice at the ballot box on November 8."

"Comey must be under enormous political pressure to cave like this and announce something he can't possibly know," Trump supporter Newt Gingrich tweeted, referring to the rapid review of the newly found emails that the FBI conducted over the past week.

"The destruction of James Comey by political pressure is painful to watch," the former House Speaker added. "He is being twisted into an indefensible pretzel of contradictions."

Another Trump adviser, former General Michael Flynn, was incredulous, sharing a view voiced by several Trump advisers that it was "impossible" that the FBI had conducted a thorough review.

"There R 691,200 seconds in 8 days," Flynn tweeted. "DIR Comey has thoroughly reviewed 650.000 emails in 8 days? An email / Second? Impossible."

Presumably to avoid giving the issue another infusion of oxygen, Clinton did not address the new letter at her rally in Ohio Sunday afternoon.

Trump's campaign manager, Kellyanne Conway, told CNN's Sara Murray that she did not believe the second letter would change the contours of the race in the final days. Even before the original Comey letter, she said more Republicans were gravitating toward their nominee, and more women and independents had begun leaning in Trump's direction.

"Our polls started to tighten prior to last Friday," Conway said. She noted that the second letter did not change Comey's harsh criticism of Clinton earlier this year.

"He still says she's reckless and careless," Conway said. "She always has something to hide. There's always a separate set of rules for her."

Trump made that argument at his midday rally in Minneapolis, shortly after the news broke.

"You have to understand it's a rigged system and she's protected," he said.