There is at least one study that support the President's beliefs on millions of people voting illegally. ... There is a Harvard rebuttal to that study...This rebuttal, you mean?
"This paper documents how low-level measurement error for survey questions generally agreed to be highly reliable can lead to large prediction errors in large sample surveys, such as the CCES. The example for this analysis is Richman, Chattha, and Earnest (2014), which presents a biased estimate of the rate at which non-citizens voted in recent elections. The results, we show, are completely accounted for by very low frequency measurement error; further, the likely percent of non-citizen voters in recent US elections is zero."
...but that doesn't change the fact there is a study supporting the President's position.
Do you realize what you're saying, here? "The study has since been revealed to have been basically bullshit. But that doesn't change the fact it supports the President's position."
It depends on the reality you're already pre-disposed to believe.
If you're already that relativist about about basic events in recent history that were attended by hundreds of thousands in person -- and witnessed on television many many hundreds of millions, worldwide -- as to basically say, "How can ya really know? It all depends on what you're pre-disposed to believe" -- then I'm afraid there's not much I can do for you, pal.