"If you're willing to believe- without any proof- that the Russians hacked the US electoral process, then why would you refuse to believe that a small percentage of US immigrants managed to cast an illegal ballot?"

The underlying question is one that few are willing to acknowledge, let alone answer. 

In a democracy, the people are obliged to critically and impartially examine every word that comes out of an elected official's mouth. But people tend not to do that. They decide with their guts first, their feelings second, and their thoughts last.