The unique base of support that has pushed Donald Trump to the top of the Republican presidential field in national polls is also likely to give him boost this spring when the campaign shifts to the South.

Sen. Ted Cruz of Texas has drawn wide attention for his own efforts in Southern states, many of which will vote as part of the big, March 1 “SEC Primary,’’ when more convention delegates will be awarded than on any other day. Mr. Cruz, who is surging in the polls, is focusing on evangelical Christian voters and other social...