WASHINGTON — Hillary Clinton and Donald Trump have been locked in a fierce election battle for months, but tens of millions of Americans will compare their presidential bona fides side-by-side anew Monday.

The first of three debates promises to be a national sensation, contrasting two vastly different New Yorkers who are recognized around the world. Clinton, known for her extensive experience in government, is more comfortable discussing substantive issues than pitching her candidacy; Trump excels as a self-promoter and an unsparing critic of his adversaries.

The Democratic presidential nominee is preparing for an unpredictable opponent who “hangs back a lot, picks his moments” and “may be aggressive,” according to communications director Jennifer Palmieri. Her challenge: driving home her message to voters regardless of what he does, Palmieri said.

The Republican nominee is being advised by some in his orbit to put his rival on defense by questioning her judgment, intelligence and accomplishments, as well as confronting her over controversies such as the Clinton Foundation, her private email server, her paid speeches to Goldman Sachs and other Wall Street firms, and by accusing Clinton and her husband of exploiting the Haiti earthquake for personal gain.

But his advisers are also wary of him going too far, and coming across as a bully.

‘HIGH-STAKES DRAMA’

The debate at Hofstra University in New York comes six weeks before the Nov. 8 election, as Trump has closed the gap nationally and in numerous battleground states, with Clinton retaining an edge in the Electoral College.

“It’s going to be a high-stakes drama,” said Peter Hart, a leading Democratic pollster. “In an hour and a half or two, opinions get suspended and people look at the candidates, on large measure, afresh. There are a certain number of open windows for people to look and decide what they’re feeling.”

“Voters who say ‘I worry that I can’t relate to Hillary’ will get an opportunity to see her. Voters who wonder if Donald Trump has the temperament or the knowledge to be president – they get to see that,” he said.

In a Fox News interview Tuesday, Trump campaign manager Kellyanne Conway cited a recent forum hosted by NBC’s Matt Lauer as a “very good preview” for what to expect from Trump during the debate. She said his answers to questions will be “concise and confident” in contrast to Clinton’s “lengthy” and “lawyerly” responses.

Clinton told donors in the Hamptons last month she’s unsure which Trump will show up to the debate: one who will try to be presidential and convey “gravity,” or one hurling insults to “score some points.”

“You have to assume, well he might approach the debate this way or he may approach it that way and he may be aggressive or he may lay back,” Palmieri said. “That’s hard to game out necessarily.”

Trump has been practicing for weeks while traveling with his top advisers. This week alone, he spent time with Newt Gingrich, Rudy Giuliani, Michael Flynn and Ben Carson, asking each of them between campaign events for their advice of potential questions for the debates, according to people familiar with his plans.

Trump aides have indicated that he isn’t practicing with mock debate sessions where someone plays Clinton. He was campaigning through Thursday, and Sunday he has reserved an entire day to prepare for the debate inside Trump Tower, according to people familiar with his plans.

Thus far, Clinton aides have declined to say who is playing Trump in her rehearsals or to say when and where she’s preparing. She was at home in Chappaqua, N.Y., all day Tuesday, and has no campaign events scheduled through the debate.

Clinton is working with the team that helped her gear up for Democratic primary debates: Karen Dunn, Ron Klain, Bob Barnett, John Podesta, Joel Benenson, Jake Sullivan and Palmieri. Barnett is standing in for Republican vice presidential nominee Mike Pence in Democratic running mate Tim Kaine’s preparations. Tony Schwartz, co-author of Trump’s most successful book, “The Art of the Deal,” is also assisting.

She has hinted that if Trump appears more restrained on stage, she’ll remind voters of the former reality TV star’s history of inflammatory comments and controversies.