Bob Corker is an important Senator from Tennessee who chairs an important senate foreign policy committee. He believes that Trump is ignorant about governing and that he is dangerous because he could trigger a world war. Trump, of course, has resorted to lying about Corker on Twitter. Corker dismisses Trump's lies, which are typical Trump responses to criticism. He also tells us that most of his Republican colleagues in the senate share his concerns about Trump. They may fear Trump's power over his political base, but they have no respect for him as a person or as our president.
Corker has decided not to run for reelection in 2018 but his position as the chair of the foreign policy committee can block many of Trump's foreign policy proposals. Trump cannot afford to lose more than two Republican votes in the senate to pass legislation. Corker is only one of many senators who could vote against Trump on important bills and appointments. Trump seems to believe that he can operate without cooperation from the Republican senate by blaming Congress for his failed legislation and by threatening them by inciting his base against those who oppose him.