President Donald Trump says Amy Acton's mom calling her a liar should be enough for Ohio voters to back Vivek Ramaswamy ... and he claims he doesn't know a thing about the rape allegations against her ex-stepfather.

POTUS spoke to reporters on the White House Lawn Friday morning ... and was asked about the recent Ramaswamy ad featuring Donna Arthur -- Acton's estranged mother -- calling her a liar and telling people not to vote for her.

A reporter then asks if he simply doesn't care about the sexual abuse allegations levied against Acton's ex-stepfather ... who was charged with raping her when she was a child. Her mother lost custody after the charge was filed.

Instead of answering the question, Trump did what he does best ... berating the NBC reporter and her network and repeating over and over that all that matters is Amy's mom thinks she's a liar.

When repeatedly asked about the rape charge, Trump says he doesn't know anything about the stepfather and doesn't acknowledge the alleged sexual abuse.

We gotta be honest ... it's preposterous to think no one on Trump's staff informed him of the controversy -- so to say he knows nothing seems fishy at best. We've reached out to the White House for more info.

Bottom line ... Trump's pushing a pro-Vivek narrative -- and anyone who has questions better prepare to be belittled.