Clinton has been hunkered down in the hotel for several days preparing for the debate, working with her closest aides to take on Donald Trump at Hofstra University on Monday. She was seen leaving the hotel on Saturday.
A longtime Clinton confidant is portraying Trump in her practice debate sessions, two people familiar with the debate preparations tell CNN.
Philippe Reines, known for his combative style and fierce loyalty to Clinton, is spending the weekend helping Clinton prepare. He has been working closely with her debate planning team for more than a month, the sources said.
Clinton is joined in the sessions by Huma Abedin, her longtime aide; John Podesta, her campaign chairman; and Jake Sullivan, Clinton's top policy aide, according to the sources.
The all-day sessions, which find Clinton sparring with multiple versions of the Republican nominee, are being led by Ron Klain, who advised President Barack Obama, and Karen Dunn, a longtime lawyer and aide.
She last appeared on the campaign trail Wednesday in Orlando, Florida. Her absence has not gone unnoticed; on Thursday, Trump asked a campaign audience where she was, remarking, "Some people think she's sleeping."
Clinton told donors last month that she isn't sure which Trump will show up on Monday: A more reserved, controlled Republican nominee or the blunt, happy-to-insult candidate voters see on the campaign trail. Because of uncertainty, Clinton is preparing for Trump to confront her on a litany of uncomfortable subjects, ranging from her email scandal to Bill Clinton's infidelity to her health.
In addition to preparing at the hotel, Clinton spent weeks reading briefing books about Trump's policy, personality and politics at her home. Aides have said that Democrats have offered considerable -- sometimes unsolicited advice -- to the former secretary of state.
"I am going to do my very best to communicate as clearly and fearlessly as I can in the face of the insults and the attacks and the bullying and the bigotry that we have seen coming from my opponent," Clinton said Tuesday on the Steve Harvey Radio show. "I can take it, Steve. I can take that kind of stuff. I have been at this, I understand it is a contact sport."
The role of Trump had been one of the most closely-guarded secrets surrounding Clinton's rigorous debate preparations. Most aides at the campaign headquarters in Brooklyn, while aware that Reines speaks frequently to Clinton, did not know he was playing the role of Trump.
The New York Times first reported
Reines' involvement in debate preparations.
Reines was a press secretary for Clinton during her Senate tenure and one of her closest advisers as secretary of state. In Clinton's memoir about her time leading the State Department, "Hard Choices," she describes him as "passionate, loyal, and shrewd."
"He usually knows what Washington's movers and shakers are thinking even before they do," Clinton wrote in her 2014 memoir.
Among aide's top concerns for Monday night are that Trump will be graded differently than Clinton, given his relative inexperience compared to the former secretary of state, who has debated upwards of 40 times, including debates in 2008 and during her Senate run in 2000.
Clinton's aides would not disclose who is playing NBC's Lester Holt, who is moderating Monday's debate, in the sessions. But they did say on Friday that they are worried about the journalist not fact-checking Trump
during the debate.
"Donald Trump has pattern of repeating lies hoping no one will correct him. Want voters to be on alert. Any candidate who tells this many lies clearly can't win the debate on the merit," Jennifer Palmieri, Clinton's communications director, said on a call with reporters. "This warrants particular focus because his level of lying is unprecedented in American politics."
Both candidates have been viewing hours of footage of their opponent to assess their debate style -- and their campaigns have strained to hang high expectations
on their rivals in an effort to minimize the impact of their own flaws.
CNN previously reported
that Wisconsin Gov. Scott Walker is playing Democratic vice presidential nominee Tim Kaine in debate preps with Mike Pence. Meanwhile, lawyer and agent Bob Barnett is playing Pence in debate prep sessions with Kaine.