Congressman Adam Schiff -- a prominent Democrat has added his voice to the growing number questioning Joe Biden's mental fitness.

The California congressman -- who's currently running for Senate -- stopped by "Meet the Press" Sunday morning to discuss the presidential race ...and, take a listen to his comments 'cause he's not sounding super confident in the sitting prez.

AS says Biden's debate performance has Americans wondering if he's up to the task of beating Donald Trump ... and, he says if it was just based on their past records, Biden would crush DJT.

Schiff says Biden's running against a "criminal" and should mop the floor with him ... before adding the president's age is keeping him from jumping out miles ahead.

The career politician says Biden needs to consult with objective people -- those outside his inner circle -- about stepping down ... 'cause he says it's essential Dems win in November.

BTW ... if you don't remember, Schiff ran the first Trump impeachment trial -- so, he's firmly in the Democrat camp with no love for the former prez. Here though, it's the head of his party he's aiming at, though he's got a few shots for Donald too.

Like we said, Schiff isn't the first Democrat to openly question Biden's mental acuity ... with many side-eyeing Biden after the debate and over comments he's made in subsequent interviews.

Comedian Rob Reiner -- a longtime liberal -- tweeted about the difficult call Sunday too ... telling his followers Biden needs to stand down or democracy itself may come to an end.

Biden's denying any sort of mental issue ... telling George Stephanopoulos he won't take a cognitive test -- and, assuring his supporters he plans to run in November.

But, if Schiff's any indication ... sounds like the Democratic party isn't 100% behind him.