With three hours to go before polls close in South Carolina, President Donald Trump is trying to sink the campaign of Rep. Mark Sanford, a Republican incumbent who’s facing a primary challenge from pro-Trump candidate Katie Arrington.

“Mark Sanford has been very unhelpful to me in my campaign to MAGA,” Trump wrote on Twitter, presumably from Air Force One on Tuesday evening. He was flying back from Singapore, where he had earlier met with North Korean leader Kim Jong Un. “He is MIA and nothing but trouble,” Trump continued. “He is better off in Argentina.”

That Argentina diss is a reference to Sanford’s going missing in 2009 to have an affair in Argentina. He was the married governor of South Carolina at the time, and he told the press and his staff that he was hiking along the Appalachian Trail.

Sanford is among the Republicans who’ve most frequently voted against Trump-backed legislation. He has voted with Trump only about 73 percent of the time, according to an analysis from 538, diverging from Trump more frequently than all but four Republicans. And, perhaps more importantly, Sanford’s been a vocal critic of Trump’s, calling for Trump to release his tax returns in 2016 in an op-ed in the New York Times. Last year he called Trump’s violent rhetoric “a problem,” and told Politico Magazine that Trump “fanned the flames on intolerance.”

Sanford’s challenger, meanwhile, has embraced Trump — and she has Sanford worried about his seat. He’s been running TV ads, which he hadn’t done in five years.

“I’m running for Congress to get things done, not to go on CNN to bash President Trump,” Arrington says in a campaign ad of her own.

It’s a race that’s being closely watched in Washington, a barometer for whether Republicans can disagree with Trump and still get re-elected. And some big-name Republicans have stuck by Sanford, including House Freedom Caucus chair Mark Meadows and House Speaker Paul Ryan.

But Trump’s weighing in on the race comes a little late. Most of Election Day had already passed by the time his tweet went out. Polls close at 7 p.m. in South Carolina.