No Republican can beat Trump in a primary. But, that's not the point. An incumbent usually runs unopposed. So they get the time off during the primaries, and the pulpit of the Presidency to "relax", while the other party does a primary fight. I saw Joe Walsh this morning, and he's not holding back. So, during the Primaries now, Trump will have to deal with him and spend his twitter angst on them and not the Democrats.