"There are a number of reasons to defeat Donald Trump, but chief among them is security," Clinton said after a long riff about how Trump's pitch to ban Muslims and other comments make it harder to work with Middle East nations and fight terrorism.
Clinton's blunt comment, while casting Trump as a risk to the nation's security, also leans into the chance that Trump could end up as the Republican standard bearer.
Trump handily won the South Carolina primary on Saturday, his second victory in his party's nominating process.
Clinton did not mention another Republican by name at the $2,700-a-head fundraiser but repeatedly hit the party writ large.
"It's just not who we are," Clinton said about the Republican Party's positions. "It's quite a troubling vision. And it's not just Trump. It's all of them."
Clinton's fundraiser was at the Studio City home of Ellen Goldsmith-Vein and Jon Vein. Goldsmith-Vein is a movie producer and entertainment executive and Vein is the co-founder and CEO of MarketShare.
The outdoor event was audible from a nearby street.
Clinton told the audience at the hilltop home that she was "feeling very good" about the state of her campaign and the race against Vermont Sen. Bernie Sanders. The former secretary of state noted her win in Nevada and the fact that she will be heading to South Carolina later this week for the state's primary on Saturday.
"We must keep the White House, and with your help that is exactly what we will do," Clinton told her supporters.
Clinton also knocked Sanders throughout her speech, using familiar lines on education, guns and Wall Street that she uses at events open to the press.
"If we don't stand up against the (gun lobby) ... shame on us," Clinton said, adding that this is a "big difference" she has with Sanders.
On banks, Clinton said, "If we had all the big banks broken up tomorrow that wouldn't break all the barriers" that are facing Americans.
Clinton did not take questions from the audience.