Spencer Pratt's allegation about Mayor Karen Bass breaking the law in one of her campaign videos could lead to a police investigation ... TMZ has learned.

Josué "Josh" Marcus with the City Clerk's Office tells TMZ they've seen Spencer's official complaint, and ... "After review, our practice is to refer these allegations to the Los Angeles Police Department for further investigation, which we will do for this and any new allegations."

Basically, the City Clerk's Office is putting the ball in LAPD's court ... asking the city's police force to decide whether the city's mayor committed electioneering in her latest campaign vid.

As we reported, Spencer accused Mayor Bass of campaigning too close to ballot boxes during early voting.

It's illegal to promote a candidate's campaign within 100 feet of polling locations, and SP thinks his political enemy did just that.

ICYMI, Spencer filed his formal complaint after Bass posted videos and photos showing campaign signs, supporters encouraging people to vote for her, and most importantly -- where the law's concerned -- a ballot dropbox.

Marcus tells us the complaint's also been passed to the L.A. County Registrar-Recorder/County Clerk's office ... which administers the June 2 primary election.

Karen's camp told us yesterday Spencer's just mad she has real supporters, instead of A.I. bots.