The Democrat's camp welcomed the news that FBI Director James Comey had decided not to change his July recommendation that she not be prosecuted for putting US secrets at risk while serving as secretary of state.
But, as both candidates criss-crossed swing states in a last desperate scramble for votes, the 11th hour decision by the Federal Bureau of Investigation refocused attention on an issue that has damaged her campaign.
"We're glad that this matter is resolved," Clinton's campaign director Jennifer Palmieri told reporters aboard the 69-year-old's campaign plane.
Late last month, with Clinton seemingly on a glide path to victory, Comey sent shockwaves through both campaigns by announcing a renewed FBI investigation in Clinton's email use.
In July, the Justice Department, acting on the FBI's recommendation, had decided not to prosecute Clinton over her decision to use a private server for official emails while she was Washington's top diplomat.
But, after a previously undiscovered batch of mails was found on a laptop belonging to the husband of one of Clinton's key aides, Comey warned lawmakers he was revisiting the case.
Trump, the 70-year-old property tycoon and Republican flag-bearer, seized on the opening, condemning Clinton's "criminal scheme" and arguing that she was unfit to be president.
Opinion polls tightened as Trump began to recover ground he lost after several women accused him of sexual assault, and the race looked headed for a photo finish.
But on Sunday, Comey wrote another letter to Congress to say that FBI agents had been working around the clock to review the "large volume of emails" found on the laptop.
"During that process, we reviewed all of the communications that were to or from Hillary Clinton while she was secretary of state," Comey wrote.
"Based on our review, we have not changed our conclusions that we expressed in July with respect to Hillary Clinton," he concluded.
'She's protected'
It was not immediately clear how the news would affect what has been one of the most bruising campaigns in modern US history, one that has the world on tenterhooks.
But there was relief among Clinton aides as they planned a huge rally in Philadelphia on Monday, the night before Election Day, starring President Barack Obama and his wife Michelle -- and Bruce Springsteen.
"We are glad to see, as we were confident we would, that (Comey) has confirmed the conclusions that he really reached in July," Palmieri said.
Trump, who has hammered away at the email issue in stump speeches three to four times a day for the last few weeks of the campaign, had no immediate reaction to Comey's decision.
But at a rally in Minnesota, a state won comfortably by Obama in 2012, he returned to his familiar theme of the corruption of the Washington elite.
"Hillary Clinton will be under investigation for a long, long time, for her many crimes against our nation, our people, our democracy," he declared.
It was not clear as he spoke if he was aware of Comey's letter as he predicted that these investigations would likely conclude in a criminal trial.
But he warned voters: "You have to understand, it's a rigged system and she's protected."
Campaign sprint
Last-minute campaign events -- including a midnight rally Monday night by Clinton -- have already been planned in the most contested states that will ultimately determine victory.
If Clinton wins, America's first female president will seek to build on Obama's cautious but progressive legacy, including his controversial health insurance reforms.
Trump, an unapologetic populist, has vowed to tear up the reform along with America's free trade agreements, to "rebuild" what he says is a depleted US military and review its alliances.
On Sunday, Clinton attended services at a majority-black church in Philadelphia, a traditionally liberal city where getting out the vote is crucial for Clinton to win Pennsylvania.
"Every hour and every minute counts," Clinton told congregants at the Mount Airy Church of God in Christ.
"We cannot get this wrong. The stakes are too high," she said, warning that electing Trump could signal "a return of some of our worst moments."
With an army of volunteers knocking on millions of doors and making phone calls in key states, Clinton is banking on star power to lock in her narrow poll lead.
She hosted back-to-back weekend pop concerts, with Beyonce and Jay Z in Cleveland and Katy Perry in Philadelphia.
She has also recruited Springsteen to join her rally with Obama in Philadelphia Monday night, hours after the president campaigns in the small but pivotal state of New Hampshire, where Trump has edged ahead in polling.
Trump has embarked on a cross-country odyssey through key battlegrounds Florida, North Carolina, Ohio and Pennsylvania.
He is also planning stops to poach once-reliably Democratic states as he treads a narrow path toward winning a majority of the 538 votes in the Electoral College.
'Last chance, folks'
Polls give Clinton a narrow national lead.
NBC/Wall Street Journal's final election poll showed Clinton with a four-point lead over Trump, 44 percent to 40 percent.
An ABC/Washington Post tracker Sunday put Clinton five points ahead. Polling averages are closer.
Respected FiveThirtyEight.com election forecaster Nate Silver has Clinton as a two-to-one favorite against Trump, but said her lead appears "less solid" than Obama's did in 2012.
"Rather be in her shoes than Donald Trump, but it's not a terribly safe position," Silver told ABC.
Clinton campaign manager Robby Mook dismissed suggestions that Clinton is trying to shore up her crumbling northern firewall.
"Donald Trump has to win all of these battleground races," he said. "If we win Pennsylvania and Florida, he just has no path."
Mook pointed to signs of strong early voting in Florida, Nevada and North Carolina among Hispanic Americans, who overwhelmingly favor Clinton.
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