Candidates need more than 50 percent of the vote to avoid the head-to-head runoff.
"Based on the number I see, I think that I'll end up losing this election", he said.
Trump's tweet just before 6 a.m. Wednesday also says Stewart has "a major chance of winning!". Then the SC governor, Sanford disappeared from the state and later admitted he was with an Argentinian woman with whom he was having an extramarital affair. He is MIA and nothing but trouble. He is better off in Argentina.
Arrington also used the sex scandal in her campaign.
As voters went to the polls Tuesday, Trump took the extraordinary step of tweeting with advice to defeat an incumbent congressman from his own party.
"Bless his heart, but it's time for Mark Sanford to take a hike - for real this time", Arrington said in the ad.
Sanford endorsed Texas Sen. She has focused her campaign on her support for Trump.
Trump was already making life hard for Sanford before that tweet.
In defense of his stance on Trump, Sanford has said he criticizes anyone regardless of party if they deserve it. He went on to attack Trump's penchant for lying and distorting facts.
Sanford was critical of Trump during the 2016 presidential campaign, and has found himself on opposite sides with the president since. State Attorney General Janet Mills is expected to win the Democratic nomination, while business executive Shawn Moody is the frontrunner in a close, four-person race for the Republican nomination. Rep. Robert Pittenger was ousted in North Carolina last month.
He will face Democratic Sen. Nevada, North Dakota, Maine and Virginia also chose candidates for the midterm election on November 6, when Democrats hope to capture a majority of the U.S. Congress.
Trump targets this SC candidate with a primary day tweet
Sanford, a member of the hard right House Freedom Caucus, has always been one of the few congressional Republicans willing to openly break with Trump.
Democrats will also be watching the outcomes of Virginia's 2nd District (GOP Rep. Scott Taylor), 7th District (GOP Rep. Dave Brat) and 5th District (retiring GOP Rep. Tom Garrett), where they hope to be competitive in the fall. In Virginia, a Republican known for his ardent defence of Confederate symbolism, Corey Stewart, won the Republican primary to face Democratic Sen.
And in Virginia's 10th Congressional District, state Sen.
In Virginia, a state legislator favored by the Virginia Democratic Party establishment won a crowded battle for the right to challenge one of the most endangered congressional Republicans in November's elections.
Polls have closed in both states and well as Maine, North Dakota and Nevada.
Meanwhile, longtime state Rep. James Smith has won the Democratic seat.
In Nevada, the parties will choose their nominees in two competitive open seats.
In the Seventh Congressional District, incumbent Republican Rep. Dave Brat will face off against the victor of a Democratic primary between former Central Intelligence Agency operative Abigail Spanberger and retired Col. Dan Ward. Another former congressman, Cresent Hardy, is the favorite to be the Republican in the race.
In the state's Second Congressional District, which includes the Hampton Roads region, Democrat Elaine Luria was the declared victor over rival Karen Mallard with 95 percent of the vote tallied.
The two primaries were sleepy Tuesday, but will be among the nation's most important battlegrounds in November. Democrats in Nevada chose Representative Jacky Rosen, a first-term lawmaker and member of the centrist Problem Solvers Caucus, to challenge GOP Senator Dean Heller.
Democrats, meanwhile, nominated state Rep. James Smith, a Joe Biden ally, to take on the Republican victor.
The use of the system angered outgoing Republican Governor Paul LePage, who told a television station it was "the most horrific thing in the world".