The first presidential debate Monday night is likely to be the most-watched in history. Hillary Clinton and Donald Trump will face each other in prime-time for 90 minutes. The television audience could be 100 million people or more.

• Presidential debate 2016: Five things to watch in Monday night's showdown
  

A national poll out Monday shows the race is tied at 46 percent. Clinton led by six points in the same Bloomberg Politics poll after the conventions.

The scene itself will be a reminder of how improbable this all once was: Donald Trump in the ultimate political ring, statistically tied with Hillary Clinton in the polls for the presidency. In terms of preparation, Trump was far more nonchalant than Clinton -- a daring calculation that could catapult or crater his candidacy, reports CBS News correspondent Major Garrett.

“Trump is doing great. He’s being himself,” Former New York City Mayor Rudy Giuliani said.

“Mr. Trump is ready for the debate. He’s very excited,” Trump’s campaign manager Kellyanne Conway said.

Trump’s surrogates glossed over the GOP nominee’s light debate prep and suggested a life of real estate and reality TV may be enough.

“I think Donald Trump has been preparing for this debate for his entire lifetime,” Trump’s vice presidential running mate Mike Pence said.

Trump has held no mock debates and devoted little time to briefing books. His recent policy speeches have all been on tele-prompter, a crutch he will not have against Clinton.

House Speaker Paul Ryan sounded a cautionary note.

“Hillary Clinton’s been doing this most of her life. She is the consummate pro. This is new for Donald. So I think he should obviously over-prepare for it,” Ryan said.

Leading up to the debate, Trump pledged to spend $140 million on TV and digital advertising between now and Election Day -- 10 times his budget so far. The $100 million TV investment, first reported by CBS News, will finance ads in 13 states, including three states Trump added because of tightening poll numbers: Wisconsin, New Mexico and Maine.

In another pre-debate development, Trump’s arch rival Ted Cruz buried the hatchet and formally endorsed Trump, citing the need to protect future Supreme Court vacancies from Clinton.

There are also pre-debate hijinks. Clinton’s campaign invited brash billionaire and Trump critic Mark Cuban to the debate as her guest.

Trump returned the psychological warfare suggesting he “will put Gennifer Flowers right alongside of him!”

Flowers revealed a decade-long affair with then-Gov. Bill Clinton, running for his first presidential term in 1992. Trump’s aides said Flowers has not been formally invited, and they do not expect her to be there.