Trump and Haley face each other in the Republican primary in Michigan, a key state for both. The former president is the only Republican since 1988 to win Michigan in the general election.
But Haley could prevail in some traditionally Republican counties that have turned their backs on Trump in the past. In this Tuesday's election, 16 delegates are at stake. But another 39 will be distributed at a state party convention next Saturday. Trump aspires to take the delegates to strengthen his candidacy and face whoever wins the Democratic race, which has been dominated by the president.