Dennis Quaid Stumps for Donald Trump in Coachella, 'Time to Pick a Side'

Dennis Quaid doesn't just play former presidents, he also campaigns for them ... stepping up to the mic during a Donald Trump rally in SoCal's Coachella Valley, and declaring it's time "to pick a side."

Fresh off his starring role in "Reagan" -- who Dennis called his favorite prez of the 20th century -- the actor sang Trump's praises before the candidate came on stage.

While wearing a MAGA cap, Quaid asked the crowd, “Are we gonna be a nation that stands for the Constitution or for TikTok?” He also compared Trump to Ronald Reagan, saying the nation is facing some of the same issues it did during the 1980 election.

Dennis attended the rally with his brother Randy Quaid, who's also a vocal Trump supporter, and called Donald ... his "favorite president of the 21st century."

The outdoor rally wasn't far from the site of the world famous Coachella music festival -- and while you probably think of that event as a liberal mecca, it is held in California's Riverside County ... a staunch conservative voting area.

Quaid's speech is noteworthy because Hollywood actors rarely show any public support for Trump. Yes, most happen to be Democrats, but those who aren't have said they fear a backlash from studios and casting directors.