President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE early Monday knocked former Rep. Mark Sanford Mark SanfordOn The Money: Business world braces for blue sweep | Federal Reserve chief to outline plans for inflation, economy | Meadows 'not optimistic' about stalemate on coronavirus deal Trump critic Sanford forms anti-debt advocacy group Republicans officially renominate Trump for president MORE's (R-S.C.) entry into the Republican presidential primary, labeling the former South Carolina governor and his two other GOP challengers the "Three Stooges."

"When the former Governor of the Great State of South Carolina, @MarkSanford, was reported missing, only to then say he was away hiking on the Appalachian Trail, then was found in Argentina with his Flaming Dancer friend, it sounded like his political career was over," he tweeted.

"It was, but then he ran for Congress and won, only to lose his re-elect after I Tweeted my endorsement, on Election Day, for his opponent. But now take heart, he is back, and running for President of the United States. The Three Stooges, all badly failed candidates, will give it a go!" he added.

When the former Governor of the Great State of South Carolina, @MarkSanford, was reported missing, only to then say he was away hiking on the Appalachian Trail, then was found in Argentina with his Flaming Dancer friend, it sounded like his political career was over. It was,.... — Donald J. Trump (@realDonaldTrump) September 9, 2019

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Sanford announced his candidacy on Sunday, joining former Massachusetts Gov. Bill Weld William (Bill) WeldRalph Gants, chief justice of Massachusetts supreme court, dies at 65 The Hill's Campaign Report: Biden visits Kenosha | Trump's double-voting suggestion draws fire | Facebook clamps down on election ads Biden picks up endorsements from nearly 100 Republicans MORE and former Illinois Rep. Joe Walsh Joe WalshSunday shows preview: Protests continue over shooting of Blake; coronavirus legislation talks remain at impasse Republicans officially renominate Trump for president Tucker Carlson responds to guest correcting pronunciation of Kamala Harris's name: 'So what?' MORE in challenging Trump for the Republican nomination.

Pressed by "Fox News Sunday" host Chris Wallace Christopher (Chris) WallaceHouse to vote on resolution affirming peaceful transition of power Gayle King calls out Pelosi for calling Trump supporters 'henchmen': 'Egregious language' GOP lawmakers distance themselves from Trump comments on transfer of power MORE on whether he believed he had a real chance in 2020, Sanford said, "You never know."

"Let’s go out and force or try and create a conversation on that which is not being talked about in this presidential cycle," he added.

Wallace also questioned Sanford about the extramarital affair that caused a scandal while he was governor.

“I learned a level of humility, a level of empathy that I didn’t have before,” Sanford said. “It is something of great regret. It’s something I’ve apologized extensively for.”

“In contrast to the president, where he says there’s not a single thing that he sort of regrets or apologizes for, I profoundly apologize for that,” he added.

Trump has also been accused of having affairs, notably with adult-film star Stormy Daniels.

Sanford, Weld and Walsh will face a president who has continued to receive support from most Republicans. A recent Morning Consult poll found that 85 percent of them approved of Trump.

The Republican parties in Kansas and Sanford’s home state of South Carolina on Saturday also voted Saturday to cancel their GOP primaries in 2020 ahead of the presidential race in moves that were criticized by the president's challengers.

Updated at 7:13 a.m.