President Donald Trump says he'd like Spencer Pratt to succeed in his Los Angeles mayoral bid ... because he's heard the candidate is a Trump loyalist, through and through.

POTUS was asked about Pratt -- a fellow reality star running for office just like DJT himself back in 2015 -- at Joint Base Andrews in Maryland on Wednesday morning ... and he says he'd like to see SP do well, since he's heard Pratt's a MAGA supporter.

Trump notes he doesn't know the reality star personally ... but as long as Pratt's rocking with him, he's rocking with Pratt.

Spencer did not exactly embrace the support in a heavily Democratic city, telling TMZ, "Everybody wants me to succeed because L.A. is the most important city in the country," adding, "The only support I need is from moms that wanna feel safe in Los Angeles. I'm laser focused on that."

The Prez notes it won't be easy for Pratt to win in California ... claiming the people who run the elections are dishonest -- and even making a strange comment about needing Jesus Christ to come down from Heaven and combat Democrat dishonesty.

We've been all over Spencer's run for mayor ... highlighting his attacks on incumbent Mayor Karen Bass and CA Governor Gavin Newsom -- both of whom are on Trump's you-know-what list.

We also learned ... if the Pratt wins, cameras will be following his every move in office ... because he, Heidi Montag, and their kids will star in a new docuseries.