Updated

Philippe Reines, a long-time Hillary Clinton aide, is playing the role of Donald Trump in preparation for Monday night’s first 2016 presidential debate, Fox News confirmed Saturday.

Reines -- who worked for Clinton on her 2008 presidential campaign, as well as when she served as a New York senator and as secretary of state -- has a reputation for being pugnacious like Trump, the Republican nominee and former reality TV star.

“Philippe does his verbal combat at a very high level. He is smart, understands the issues and understands the back and forth of political debate," said the source who confirmed Reines’ involvement for Fox News.

The story was reported first by The New York Times, which on Saturday, as expected, endorsed Clinton, the Democratic nominee, for president.

The newspaper's editorial board on Saturday praised Clinton for bringing "a record of service and a raft of pragmatic ideas" to the election. It calls her "one of the most tenacious politicians of her generation, whose willingness to study and correct course is rare in an age of unyielding partisanship." Donald Trump is described as the worst nominee put forward by a major party in modern times.

The Times has endorsed only Democrats for president back to John Kennedy in 1960.

The Fox News source described Reines as a combative and blunt guy who would have no problem going after Clinton’s perceived weaknesses and trying to get under her skin to see how she would handle Trump's likely attacks.

Clinton and Trump will participate Tuesday night in a 90-minute, prime-time debate hosted by NBC-TV at Hofstra University, in Long Island, N.Y.

The nominees are essentially tied with about six weeks to go before Election Day, according to most polls.

Their second debate will be Oct. 9 at Washington University, in St. Louis, and the third will be Oct. 19 at the University of Nevada, Las Vegas.

On Sunday, Clinton was purportedly off the campaign trail to prepare for the debates, while Trump is scheduled to hold an evening rally in Roanoke, Va.

Trump needs to prove to voters that he has the policy depth and gravitas to serve as commander in chief. Clinton needs a moment to connect with Americans who question whether she can be trusted.

Both campaigns expect a record-setting television audience for the high-stakes showdown, which could help tip the balance in the tight

According to a new Associated Press-Gfk poll, more than 85 percent of likely voters backing Clinton or Trump say their minds are completely made up. About 13 percent said they were undecided.

Their respective preparations have been a microcosm of their sharply different approaches to politics and presumably, the presidency.

Clinton has spent weeks with advisers, taking full days away from campaign travel to pour through briefing books, practicing to pounce if Trump makes false statements and steeling herself for the possibility that he levels deeply personal attacks.

Trump has eschewed traditional debate preparations but has held mid-flight policy discussions with a rotating cast of advisers. He's also spent numerous Sundays batting around ideas with aides.

The Republican businessman's loose approach is potentially risky given that he is new to many policy issues expected to come up during the debate. But advisers contend he will compensate by being quick on his feet and point to his experience at performing under pressure.

"Imagine the practice and the training of 13 years of reality television on `The Apprentice' and then imagine Hillary's experience reading hundreds of papers," said Newt Gingrich, the former House speaker and a Trump adviser who has been talking through policy with the candidate in recent days.

Clinton aides fear Trump will indeed be judged more for his performance than his grasp of the numerous challenges that pass across a president's desk. They've been flummoxed by Trump's ability to sail through the campaign without fleshing out many policy positions and glossing over past statements that he no longer views as politically palatable.

On Friday, the Clinton campaign released 19 pages of what they called Trump's "seven deadly lies," including his false assertion that he opposed the Iraq war from the start.

Clinton has debated more than 30 times at the presidential level, including several one-on-one debates with Barack Obama in 2008 and Vermont Sen. Bernie Sanders in 2016. But this will be her first presidential debate against a candidate from an opposing party, which Democrats say will give her far more latitude in drawing a contrast and defending herself.

Trump had an uneven record during the Republican debates, sometimes controlling the crowded contests and other times fading into the background. He flipped back and forth between being bombastic and trying to act restrained.

Trump's less-than-restrained side was on display Saturday as he took to Twitter to criticize Clinton's decision to ask businessman Mark Cuban -- a frequent Trump critic -- to be one of her guests at the debate. The Republican nominee suggested he might put Gennifer Flowers, a woman who had a relationship with Bill Clinton, "right alongside" Cuban.

The Associated Press contributed to this report.