Trump violated the enulments clause the day he took office and continues to do so.  That's an impeachable offense.  If the dems win the senate in 2018, he will be impeached because of that

HOWEVER - the bigger threat is the Russian story.  If proof emerges that his campaign colluded, the Republicans will impeach him themselves.  The majority of elected GOP would prefer PEnce over Trump anyways.  They just need an excuse to impeach or they risk the wrath of their voters.  But something like an FBI bombshell would give them the cover they need to do it.