Donald Trump is all in on Sydney Sweeney’s controversial American Eagle ad campaign … and it’s all because of her voter registration.

The prez was taking questions from reporters Sunday evening — when one brought up that Sweeney had been revealed as a registered Republican over the weekend.

Trump seemed surprised to learn the actress was part of his political party … responding, “Now I love her ad.”

He follows up, “You’d be surprised at how many people are Republicans. That’s what I wouldn’t have known but I’m glad you told me that.”

Then he doubles down … “If Sydney Sweeney is a registered Republican, I think her ad is fantastic.

Clearly, DT is well aware of the stir surrounding American Eagle’s “Sydney Sweeney Has Great Jeans” campaign … which critics on social media have argued is filled with implicit racism.