They can't prove it. That is the whole thing. This is an effort to humiliate the President as he is about to have an important meeting with Putin. I hope the two leaders drink a lot of vodka and collude/think about how to release both Mueller and Rosenstein from being slaves to lame propaganda so that they can find employment better suited to their mindlessness such as dancing on a stripper pole and cavorting with Stormy G-string.
The only reason the President would be "humiliated" is because of his own foolish denial of what his Intelligence Services have been telling him with undeniable evidence for over a year and a half. Even the Republicans in Congress who have been read in believe it.
If Trump wants to continue to be wrongheaded about this, he deserves to be embarrassed.