South Carolina Republicans failed to settle their special U.S. Senate primary outright on Tuesday, sending Sen. Darline Graham — the sister of the late Sen. Lindsey Graham — into an Aug. 25 runoff after she fell short of the majority needed to win the nomination outright.

Graham was appointed by Gov. Henry McMaster to finish her brother's term after Lindsey Graham, a 23-year fixture of the Senate, died suddenly at age 71 last month, according to CNN. His death triggered a compressed special-election calendar under South Carolina law, with the primary landing barely a month later.

President Trump had endorsed Graham's bid to keep the seat in the family, but she now faces a runoff against a rival from a crowded GOP field that also included Reps. Ralph Norman and Russell Fry and former Gov. Mark Sanford, according to NPR. The race for the second runoff spot between Norman and Fry was too close to call Tuesday night.

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Norman, a fiscal conservative who placed third in June's gubernatorial primary, has picked up endorsements from Sens. Mike Lee and Rick Scott. Fry, a member of the House Freedom Caucus, is running despite Trump's endorsement of Graham — a notable break from a president who has otherwise dominated the state's Republican politics this cycle.

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Whoever emerges from the Aug. 25 runoff will be heavily favored in the general election against Democratic nominee Annie Andrews, a pediatrician who has centered her campaign on healthcare, abortion rights and gun safety, in a state Trump has carried comfortably in each of the last three presidential elections.

The eventual winner will serve out the remainder of Lindsey Graham's unexpired term, inheriting a seat he held for nearly a quarter-century, before facing voters again in a future general election.

South Carolina's special primary was one of several contests across the country Tuesday, with voters also casting ballots in primary races in Alabama, Minnesota and Wisconsin. Turnout in South Carolina's race was watched closely as an early gauge of Republican enthusiasm heading into the fall, given the unusual, compressed timeline forced by Graham's sudden death and the emotional weight of a family member seeking to hold his seat.

Both national parties are treating the eventual matchup as a test case for how South Carolina Republicans handle a post-Graham era, with the winner inheriting not just a Senate seat but the political network he spent more than two decades building.