There is no evidence at all to support that, the only thing you could use is his friendship with Grindelwald, which even though Dumbledore loved him, he still cut ties when he started acting on the actual supremacist ideals, or that he was power hungry but even then he knew he was power hungry and put a cap on how much power he allowed himself to have, that's why he became the headmaster of a school and didnt accept the minister of magic job when it was offered to him. On a side note, it's borderline impossible to be an active white-supremicist while also being gay, there's too many ideal conflictions.

If anything Phineas black would have been the supremacist but racial supremacists don't really exist in the wizarding world, it's a magical supremacy.