The president seemed more focused on his own rivals than on McMaster's -- or on simply endorsing the governor.
Some attendees told ABC News they didn't support McMaster but stood in line in 92-degree heat just to see Trump.
The president discussed immigration reform and worked to justify his tweet and comments Monday in which he shunned the constitutional protections provided by due process.
"So if a person comes in, steps one foot, they take their name, bring them to court, release them, go into the country. You never see them again. It's the craziest thing I've ever seen," Trump said. "So I said today, I don't want judges. I want ICE and border patrol agents. That's what I want. That's right. And we want to tell people, 'I'm sorry you're coming in to the country illegally. We don't want you in the country. That's it. It's now over.'"
Trump took the stage around 8 p.m. to a raucous ovation from several thousand supporters who endured severe thunderstorms and a one-hour delay before the president began his remarks.
"It was a little rocky up in the sky tonight," Trump said of his delayed arrival, revealing that an aide asked him to consider canceling the appearance, and he responded: "There's no way. We cannot stop."
Rather than focus his remarks on McMaster or the state's governor's race, Trump lobbed red meat to his supporters, who repeatedly heckled CNN correspondent Jim Acosta even before the president arrived and criticized journalists as "fake newsers."
"Now you know we have a lot of fake news back there -- the fake news," Trump said, drawing a chorus of boos aimed at the press. "You know that if a horrible thing happens, and we weren't lucky enough to have Henry win, you know they won't talk about it. They'll say, 'Donald Trump suffered a major, major defeat in the great state of South Carolina. It was a humiliating defeat for Donald Trump.' So please get your asses out tomorrow and vote."
Trump continued to bash Hillary Clinton, igniting the crowd into a frenzy. He raved about his strained relationship with Canadian Prime Minister Justin Trudeau. He called the Democrats "the party of Maxine Waters and Nancy Pelosi."
"They want open borders," Trump added. "We want really tough borders."
McMaster ascended to the governorship after the president appointed former South Carolina governor Nikki Haley as his ambassador to the United Nations.
'She's going to be 100 percent'
Earlier this month, Trump intervened in a Republican primary when he threw his support behind Katie Arrington. She was challenging incumbent Rep. Mark Sanford, "a guy I never liked too much," Trump said, and she ended up winning. Her campaign will be delayed, however, after she was in a serious car accident last week.
"Let's all take a moment to send our love to Katie Arrington," Trump said. "Katie is a very special person. She was out there from the very beginning, and here's the good news: She's going to be 100 percent."
Trump also talked about Kim Jong Un.
"People didn't want to be nuked in the stadium watching the opening ceremonies," Trump joked.
'If it's not your hair, don't run for office, folks'
Trump also talked about Jimmy Fallon, saying that the "Tonight Show" host's impression, which includes an orange hairpiece, isn't on point.
"The guy screws up my hair," Trump said. "Everybody used to say my hair is phony. It's not my hair. I'm wearing a hair piece. Anybody here wearing a hair piece now? No? But the one thing -- they never said that anymore. I've been caught in rainstorms, winds 60 miles an hour. If it's not your hair, don't run for office, folks. Don't run."
After about an hour of talking about his own accomplishments as president, Trump finally turned back to McMaster and said to the crowd: "We will make America great again."