Sen. Darline Graham advanced Tuesday to a runoff in South Carolina's crowded Republican Senate primary, extending the fight to succeed the late Sen. Lindsey Graham. Her opponent is not yet determined. Why it matters: The Aug. 25 runoff will almost certainly determine South Carolina's next senator. President Trump endorsed Graham shortly after she formally entered the race. The eventual Republican nominee will be heavily favored against Democrat Annie Andrews in November in one of the nation's most reliably Republican states. Democrats have not won a U.S. Senate election there since 1998. Driving the news: No candidate won the majority required to secure the Republican nomination outright, according to AP. The big picture: The result narrows a sprawling nine-candidate contest to a head-to-head fight among the race's three dominant forces. Graham, 62, has cast herself as a political outsider who can continue her brother's constituent-focused work while serving as a reliable vote for Trump's agenda. As votes were counted Tuesday, Rep. Ralph Norman (R-S.C.) and Rep. Russell Fry (R-S.C.) were close behind. Norman, 73, has argued that Republican voters should judge candidates by their records, presenting himself as a proven conservative rather than a first-time candidate elevated by a famous name. Fry, 41, has presented himself as a next-generation conservative with a congressional résumé, an existing campaign organization and experience winning a Trump-aligned Republican primary. The intrigue: The runoff will offer a revealing test of Trump's influence over a Republican electorate in which nearly every major candidate supports his agenda. What's next: The two finalists have 14 days to consolidate the supporters of the eliminated candidates.