In media mythology, the first 100 days is make-or-break time for a new president and an easily packaged time frame that is, of course, inherently arbitrary.

While the phrase conjures up an FDR-like New Deal, it can also be a tough slog as a new president tries to push legislation through Congress, get nominees confirmed and becomes enveloped by Beltway process stories.

But given Donald Trump’s unique status as an outsider who ran against the establishment of both parties, the first 100 days could be more fiery than usual. And that’s what Martha MacCallum is banking on.

“We have to hold his feet to the fire,” she told me in a phone interview.

The “America’s Newsroom” co-host is launching a new show Monday at 7 p.m. ET to chronicle the debut of the Trump administration.

MacCallum, a onetime Wall Street Journal staffer and stock exchange reporter for CNBC, says the business community is counting on Trump slashing regulations and cutting taxes. “That in many ways is what the market is reading,” she said. “He needs to make sure that’s one of his top priorities.”

MARTHA MACCALLUM SET TO PREMIERE NEW PRIMETIME SHOW 'THE FIRST 100 DAYS'

On Sunday's "Media Buzz," MacCallum said that "the question has hung out there" since the election: "Will he become more presidential when he takes that oath of office?" As for the controversies that surround his statements and misstatements, she said: "I think in a strange way there’s an acceptance of exaggeration with Donald Trump that we haven’t had with former presidents."

A political science major at St. Lawrence University in upstate New York, MacCallum says politics -- and history -- have always been her passion. She paid some serious dues, waitressing at four Manhattan restaurants while working as a fact-checker at a financial magazine. She also “got bitten by the theater bug,” studied acting and directing in graduate school and worked with a small theater company -- which may have helped her future career.

“At the time I would have scoffed at any connection between the pure art form of theater having any relation to the news,” she says. “But in both it’s about connecting with your viewers…To be a good waitress is an art. You’ve got to juggle a lot of tables and people.”

The reason she’ll be serving up the new fare at 7 p.m. on Fox is that the slot opened up when Megyn Kelly decided to leave for NBC and Tucker Carlson’s new show was moved to 9 p.m. MacCallum, who has three children, isn’t sure she’ll continue in the evening -- she loves her morning partnership with Bill Hemmer -- but she is pumped about raising the curtain on the Trump presidency.

“We have seen plenty of presidents get gobsmacked in the first 100 days,” she says.