Trump threatens a lawsuit here:
“One of the ways I can fight back is to bring a lawsuit against him relative to the fact that he was born in Canada and therefore cannot be President. If he doesn’t take down his false ads and retract his lies, I will do so immediately. Additionally, the RNC should intervene and if they don’t they are in default of their pledge to me.”
https://www.facebook.com/DonaldTrump/posts/10156647104280725
Were he a serious candidate and were he ever to be electable, his support from any congress would be next to nil.