At a glance, President Trump appears to have a knack for selecting successful primary candidates. Of the hundreds of Republicans he has endorsed in this election cycle, an overwhelming majority have coasted to victory. In perhaps the toughest test of Mr. Trump’s influence yet, Senator Darline Graham prevailed in South Carolina on Tuesday, extending a primary winning streak that Mr. Trump routinely boasts about.

Superficially, the math supports his claims. But by favoring mostly incumbents and candidates running in uncontested races, Mr. Trump’s primary wins are built largely on low-risk bets.

This cycle, Mr. Trump has endorsed at least 260 primary candidates running for House, Senate or governor. So far, only nine have lost their races.

Five of these Trump-endorsed candidates have not yet had their primaries. Two are in New Hampshire, which holds its primary next month. Three are in Louisiana, whose governor pushed back the state’s House primaries to November to make time for redistricting.

Nearly 40 percent of the candidates Mr. Trump endorsed have run in uncontested primary races, all but ensuring their spots in the general election.

The other 159 Trump-endorsed candidates ran or will run in contested primaries. About three in five people in this group were incumbents, who typically benefit from name recognition and well-resourced campaigns. The rest were running against incumbents or for open seats.

Outside the sanctuary of incumbency, challengers and open-seat candidates are typically riskier bets.

Indeed, seven of the Trump-endorsed candidates who lost their primaries were in this group. Two House candidates, Amir Hassan in Michigan and Catalina Lauf in Florida, lost their primaries. And five gubernatorial candidates, Randy Feenstra in Iowa, Burt Jones in Georgia, Pamela Evette in South Carolina, Mike Lindell in Minnesota and Megan Degenfelder in Wyoming, will not move on to the general election.

But not all incumbent endorsements are guaranteed wins. Mr. Trump’s other two losses so far were House incumbents. Representative Cory Mills of Florida, whom Mr. Trump endorsed in February, lost his primary while facing allegations of assault and threatening behavior toward women, which he has denied. Many Republicans had called for Mr. Mills to step aside and backed his challenger, Ryan Elijah, who won the Republican nomination. After the primary, Mr. Trump said he had told Mr. Mills to drop out.

Representative Andy Ogles of Tennessee, another incumbent with Mr. Trump’s support, faced backlash over anti-Muslim and anti-immigrant remarks. Mr. Ogles lost his primary to Charlie Hatcher, a well-funded challenger who had the support of Tennessee’s Republican governor.

Every Senate candidate Mr. Trump has endorsed has won his or her primary so far, incumbents and challengers alike. Two Trump-endorsed Senate challengers won against incumbents: Representative Julia Letlow beat Bill Cassidy, a two-term senator from Louisiana, and Texas’ attorney general, Ken Paxton, beat Senator John Cornyn, who has been in office for four terms.

Ms. Graham won her primary runoff in South Carolina on Tuesday with Mr. Trump’s endorsement, having never held public office before being appointed to serve out the term of her brother, Senator Lindsey Graham, after his death last month. She was one of the candidates Mr. Trump had most enthusiastically supported. In an unusual show of effort, he traveled to South Carolina last week to rally supporters, and he hosted a virtual rally for her on Monday night. MAGA Inc., a super PAC run by Mr. Trump’s advisers, invested more than $800,000 in Ms. Graham’s race.