Hawzah News Agency (South Carolina, US) - A few hours before polls closed, Trump tweeted that Sanford was "nothing but trouble" and "very unhelpful to me." He backed Sanford's pro-Trump challenger, state legislator Katie Arrington, for November's congressional elections.
Arrington, who made a campaign issue of Sanford's criticism of Trump, won 50.6 percent of the vote to Sanford's 46.5 percent with almost all ballots counted. That just crossed the 50 percent threshold to avoid a runoff later this month between the top two contenders.
The South Carolina race highlighted primary voting in five states on Tuesday. Nevada, North Dakota, Maine and Virginia also chose candidates for the midterm election on Nov. 6, when Democrats hope to capture a majority of the US Congress.
The upset of Sanford, a member of the conservative House Freedom Caucus, was the latest sign of Trump's firm grip on the Republican Party. The onetime insurgent has made allegiance to his leadership a litmus test in many Republican races.
Sanford had been critical of Trump at times, saying he "fanned the flames" of intolerance and decrying his disregard for facts. But during the campaign, he ran ads saying how often he voted with the president.
That was not enough for Trump, who also mocked Sanford with a reference to a 2009 scandal when the then-governor disappeared for days before surfacing to say he was "hiking the Appalachian Trail." Later, Sanford admitted he was involved in an extramarital affair in Argentina.
Trump, who issued his tweet as he returned from his summit with North Korean leader Kim Jong Un, said Sanford "is better off in Argentina."
Republican Representative Justin Amash, Sanford's fellow conservative and House Freedom Caucus member, fired back at Trump after his attack on Sanford.
"Unlike you, Mark has shown humility in his role and a desire to be a better man than he was the day before," Amash said on Twitter.