Clinton's traveling press are not typically permitted to hear her remarks at fundraising events, but were able to hear her at this event, while they waited in the basement of her donor's opulent home.
Clinton mused that Trump might seek to look or act presidential in order to "to convey a gravity." Or, she noted, he might seek to "score some points."
"This is the most unpredictable election season that I certainly can remember," Clinton said, pledging not to take anyone, anything or any place for granted in the election.
The fundraiser was held at the home of software company chief executive Charles Phillips and his wife Karen. Phillips is a native of Little Rock, Ark.
Former president Bill Clinton also attended the event.
Clinton is in the midst of a multi-week fundraising blitz that has taken her to dozens of high-dollar events in recent days and will probably result in a record fundraising haul this month.
At the gathering, Clinton also issued dire warnings about Trump, telling her supporters that he had already damaged America's standing in the world.
"We have shaken the foundations of belief in America’s reliability in too many places," Clinton said. "So we have got to repair the damage that has been done."
Recalling her speech in Reno, Nev., last week dedicated to the so-called "alt-right" figures who are closely associated with white nationalism and have embraced Trump, Clinton noted that people like former Ku Klux Klan grand wizard David Duke is staking his political campaign for the Senate on Trump's popularity.
“I wanted to pull it out from under the rock so people could see it and understand that David Duke has endorsed Donald Trump, he is the former grand wizard of the Ku Klux Klan,” Clinton said. "And nobody knows how well he is doing and how his embrace of Trump and Trump’s acceptance of him could put that man, that despicable man, in the Senate of the United States."