There's a ton of evidence. There's so much evidence that the FBI was able to present some evidence to a judge and secure two FISA warrants and a no-knock search warrant. In addition, many of Trump's inner circle will likely be prosecuted for failing to disclose foreign contacts (the evidence of this is that they had plenty of foreign contacts and omitted them every time). Many of Trump's close associates (Kushner, Flynn, Trump Jr. etc.) will be prosecuted under NY state statutes because Bob Mueller is sharing his findings with the AG of NY (about offenses that happened in NY)
As for the base sticking with him, they are turning on him en masse. Trump just struck a deal with democrats and flip flopped on the wall (a central campaign promise), wants to keep DACA after promising for two years to end it, is thinking about supporting Bernie Sanders' universal healthcare, and has gotten rid of Steve Bannon and other hardliners. Keep in mind, the republican party was deadest against Trump from the beginning and the moment he starts working with democrats, they will fully turn on him.
