Charleston, S.C. — Donald Trump is spending a lot of political capital trying to prove he can defeat any Republican who dares to challenge him. He probably sees that as a key way of asserting his dominance over the GOP. But how likely is he to succeed this year?

So far, Trump has endorsed eight GOP challengers over sitting House Republicans. Two of his incumbent targets are in South Carolina — Representatives Nancy Mace and Tom Rice — and that explains why Trump held a rally there on Saturday. “Dump these grandstanding losers,” Trump roared, referring to Mace and Rice, “and replace …