Trump says South Carolina Rep. Sanford 'better off in Argentina,' endorses primary challenger

President Trump urged South Carolina Republicans to vote U.S. Rep. Mark Sanford out of office Tuesday, tweeting that the three-term congressman was "better off in Argentina" -- a reference to a sex scandal from Sanford's tenure as governor.

Sanford, an outspoken Trump critic, is facing a tight race against State Rep. Katie Arrington, a relative newcomer to politics.

"I fully endorse Katie Arrington for Congress in SC, a state I love," tweeted Trump, who was traveling aboard Air Force One after his summit meeting with North Korean strongman Kim Jong Un. "She is tough on crime and will continue our fight to lower taxes. VOTE Katie!"

TRUMP-BASHING SOUTH CAROLINA REP. SANFORD IN TOUGH PRIMARY FIGHT AGAINST GOP NEWCOMER

By contrast, Trump said that Sanford "has been very unhelpful to me in my campaign to [Make America Great Again]. He is MIA and nothing but trouble. He is better off in Argentina."

In June 2009, Sanford disappeared from public view for six days and later claimed that he had been hiking on the Appalachian Trail. He later admitted that he had in fact traveled to Argentina to meet a woman with whom he was having an extramarital affair.

Sanford was censured by the South Carolina House of Representatives, but served out the remainder of his second term. In 2013, he won a special U.S. House election in South Carolina's 1st District, which he previously represented for six years.

Sanford endorsed Texas Sen. Ted Cruz during the 2016 election campaign. He has repeatedly called on Trump to release his income tax returns and stated that he does not have "the "stomach for [Trump's] personal style and his penchant for regularly demeaning others."

Michigan Rep. Justin Amash came to Sanford's defense on Twitter, saying that Sanford's "job is to support and defend the Constitution."

"And unlike you, Mark has shown humility in his role and desire to be a better man than he was the day before," Amash added.

Arrington is accusing Sanford of seeking the spotlight and impeding Trump's agenda. Her campaign is airing a television advertisement that refers to Sanford as a "Never Trumper" and features video clips of Sanford's Trump criticisms.

"It is time to turn the page on the Sanford era," Arrington told Fox News earlier this week. "We have had it with career politicians. That’s why we elected Trump."

Fox News' Brooke Singman and The Associated Press contributed to this report.