Many Americans are dreading a Trump-Biden rematch, but no one feels the anguish quite like a Nikki Haley voter.“She would make a great president, and the alternatives are not appealing,” said Patti Gramling, 72, before voting in the South Carolina Republican primary in February in an upscale suburb of Charleston, S.C. “Biden is too old. And I think Donald Trump is horrible.”With Ms. Haley expected to end her 2024 campaign, a crucial new equation is emerging in the electoral math: Where will her voters — and voters like them in key battlegrounds across the country — go in a…