Eva Longoria's getting political ... organizing an event aimed at mobilizing Arizona's Latina vote for the Biden-Harris campaign -- and, focusing on an old enemy to do it.

The Biden-Harris campaign tells TMZ ... the actress-director's holding an event in Phoenix on Monday where she plans on speaking to moms and young Latinas about the importance of voting -- specifically, voting for JB and KH to keep the White House.

We're told the event will place a particular emphasis on stopping Donald Trump, who the campaign says has demonized Latinos throughout his political career and is pursuing an agenda that would be particularly devastating for Latinas and their families.

It's just the latest campaign effort at mobilizing the Latino vote in AZ ... the campaign team put on events with CA Congressman Robert Garcia just last weekend for a similar purpose.

EL's been a long-time supporter of President Biden, BTW ... remember, just last year she screened her feature-length directorial "Flamin' Hot" at the White House -- so they're tight.

This will be one of the first major celeb cameos on the Biden campaign trail ... and we imagine more will follow soon enough as we approach November ... which will be here sooner than you can blink.