Diddy‘s lawyers better not bully the prosecution’s very pregnant star witness when she takes the stand next week … so admonishes Nancy Grace!

Cassie Ventura‘s upcoming testimony became an explosive topic on our new Tubi weekly series, “TMZ Presents United States vs. Sean Combs: Inside the Diddy Trial.”

Nancy appeared with Harvey, Mark Geragos, Ashleigh Banfield and Eboni K. Williams as they discussed the optics of grilling Cassie about the freak-offs for which Diddy is being tried.

Eboni K. Williams made the point, it’s a hard sell to a jury that a married, visibly pregnant mother of 2 — soon to be 3 — is a swinger who uses baby oil for things other than infants.

Geragos dared to suggest the defense will make the case the accusers are ex-girlfriends who shared mutual feelings with Diddy, and that set Nancy off.