The Oscar-winning producer didn't actually use Trump's name in the letter but referenced the billionaire's inflammatory campaign rhetoric and slogan.
"If you listen to the loudest voices out there today, you'd think we're a country that hates Mexicans, hates Muslims and thinks that committing war crimes is the best way to make America great again," he wrote. "The truth is that the only thing that would prevent America from being great would be to empower these voices."
Clooney's words come on the heels of him dubbing Trump an "opportunist" and a "xenophobic fascist" in an interview with the Guardian earlier this month.
"In all of this clutter, there's been one consistent voice — a voice of tolerance and experience from a candidate who's spent a lifetime fighting for the rights of the less fortunate. A candidate who knows firsthand the complexity of our international relationships. That candidate is Hillary Clinton," he continued in the letter.
The Oscar-winning producer went on to bolster the former secretary of State as the "only grown-up in the room" if she garnered "an insurmountable delegate lead" and Democratic nomination in the primary process.
The April 16 event — hosted at the actor's Los Angeles home with director
Clooney and the Clinton campaign are also holding a contest for supporters to meet the actor, his wife and the former First Lady during the event.
"The good news is," Clooney said, "I'm not cooking."
The "Hail, Caesar!" star also stumped on behalf of President
Clooney isn't the first celebrity to lend his voice to the Clinton campaign in the past few days: "Girls" star
Meanwhile, "Charles in Charge" alum Scott Baio endorsed Trump during a Saturday appearance on Fox News.
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