Donald Trump's encouraging his supporters to stop people on Election Day that they suspect might be up to no good ... and he's using fecal descriptions to send his message.

The ex-Prez was out campaigning in Iowa Friday -- which is about to kick off its caucus voting on Jan. 15 -- and he made no bones about what he wants MAGA loyalists to do during this presidential election in November ... put a halt to the BS, emphasis on s***.

Check out the clip ... DT ominously tells the crowd that he expects them to roam around election centers and keep an eye out for "bags of crap" that might try to infiltrate.

If the MAGA voters see any of that ... he encourages them to put a stop to it right there at the booths. On its face, it almost sounds like he's cosigning some physical confrontations.

Poop jokes aside, this is pretty serious ... and it comes across as a warning of conflicts to come as we get closer to voting day itself. Already, it's sounding like Trump is stoking the flames of election interference/tampering -- and he's riling up his base right now in January.

Of course, we've seen that story before and how it played out ... namely, Jan. 6, 2021.

It's, perhaps, no coincidence then that Trump was talking this way on the eve of the 3-year anniversary of that. And yes, he continues to insist the 2020 election was "stolen."

Remember, he's the odds-on favorite for the GOP nomination ... so it's shaping to be a rematch between him and Joe again. Here's hoping some ugly history doesn't repeat itself.