Columbia, South Carolina - Donald Trump's preferred candidate won South Carolina's Republican Senate runoff Tuesday, after a string of primary defeats raised questions about his grip on the party.

Darline Graham, the political novice appointed to replace her late brother Lindsey Graham, defeated veteran Congressman Ralph Norman after Trump invested heavily in the contest, campaigning in the state and urging supporters to vote as though his own name were on the ballot, according to projections from CNN and NBC.

Her victory interrupts a bruising run for Trump, whose endorsed candidates had lost 10 Republican contests this cycle, including six in the previous two weeks, even as his own approval ratings fell sharply.

Trump had insisted days earlier that his endorsements were "stronger than ever" and dismissed recent losses as acts of loyalty to candidates he knew faced difficult races.

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Trump traveled to South Carolina to campaign for Graham and held a telephone rally on the eve of the vote.

The Trump-supporting MAGA Inc. political organization also spent more than $827,000 on voter calls and text messages – a rare deployment of its reserves in this year's primaries.

"So what I really want you to do is pretend, please, that I'm on the ballot," Trump told supporters at a rally Friday.

After Tuesday's win, Graham is now set to face Democratic nominee Annie Andrews in the November 3 general election.

Trump's influence was also being tested in Oklahoma, another Republican stronghold, where his favored candidate Mike Mazzei won the party's gubernatorial nomination over state attorney general Gentner Drummond.

Drummond narrowly finished ahead of Mazzei in the first round in June, with both taking around a quarter of the vote. More than $9 million has since been spent on the runoff, according to various media outlets.

Trump had repeatedly attacked Drummond as a "fake Republican," in part because the attorney general opposed a proposed $4 billion aluminum smelter backed by the president, arguing that the project could damage local land and communities.

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Mazzei, a wealthy former state senator, publicly embraced the project shortly before Trump endorsed him.

The Republican nominee is up against Democrat Cyndi Munson in the general election in November.

Before Tuesday, Trump's primary record was battered by recent defeats in Florida, Wyoming, Minnesota, and Tennessee, among others.

Any further losses would have added to questions over whether Republican voters remain as willing to take his direction when Trump himself is not on the ballot.