The Republican nominee for president hit New Jersey Saturday for a rally -- the first he's had in a few weeks 'cause he's dealing with his criminal trial in NYC -- and, he started discussing immigration ... specifically the kind of people who immigrate.

Check out the clip ... DT claims other countries are opening up their mental institutions and sending unstable people to the U.S.A. -- claims about immigrants he's sorta made in past speeches.

Then, the former prez sorta goes off the rails ... asking if the crowd's seen the hit 1991 movie "The Silence of the Lambs" -- and he calls "the late great Hannibal Lecter" a "wonderful man."

Trump then jumps back on the immigration convo ... finishing off by saying there are just certain people -- unvetted and unwanted -- flooding into the country.

On its face, it seems like Trump's saying undocumented immigrants could end up being serial killers like Hannibal Lecter ... though the whole tangent's so bizarre it's hard to tell if that's totally what Trump meant.

Donald's campaign event comes after a couple weeks of testimony in his New York criminal trial ... and, key witness and former alleged fixer Michael Cohen's set to testify against his former boss Monday.