“Shame on Lindsey Graham,” said Walsh, who is running for the 2020 Republican presidential nomination against President Donald Trump, of the Republican senator from South Carolina, one of Trump’s strongest defenders in Congress. “There are so many Republicans in the House and the Senate who have sold their soul to this President. Lindsey is just one of them,” Walsh told CNN’s Gloria Borger at the CITIZEN by CNN conference in New York. Former South Carolina Gov. Mark Sanford, who is also running for the Republican presidential nomination, charged at the CITIZEN conference that Graham’s support of the President was political.”I think he’s been transparent in saying this is in essence the cost of admission for staying relevant and therefore I’m willing to pay the price,” Sanford said.Read MoreSanford lost his primary race last year for South Carolina’s 1st Congressional District after he failed to find backing in the state’s Republican Party as a vocal critic of the President.Graham was a harsh Trump critic when he was running against him for the Republican nomination in 2016 — he said at the time that Trump didn’t have the “temperament or judgment to be commander in chief” — but has since changed his tune and become an ardent supporter.McCain, who died in 2018, was one of the Republicans who were more critical of the President. He called Trump’s news conference with Russian President Vladimir Putin, when Trump sided with Putin over the US intelligence community, “disgraceful.” The Arizona Republican said in a statement after the news conference: “The damage inflicted by President Trump’s naiveté, egotism, false equivalence, and sympathy for autocrats is difficult to calculate.”CNN has reached out to Graham for comment.Walsh and former Massachusetts Gov. Bill Weld — also onstage Thursday and running for the Republican presidential nomination — said that under no circumstances would they vote for Trump in 2020. Sanford said he wouldn’t want to vote for Trump, but he didn’t rule it out. Walsh and Sanford said the most dangerous Democrat for Trump in 2020 would be Sen. Elizabeth Warren of Massachusetts, who has solidified her status at or near the top of the polls. Walsh said Warren has the “energy and the fire,” and Sanford said she is “articulate, and bright and quick.”Weld said he’d vote for former Vice President Joe Biden over Trump, and that he sees Biden as a bigger threat to the President. “Could I vote for a Democrat? Hell, yes. If it’s Trump against Joe Biden, I’m with Biden in a heartbeat,” he said. He said Warren “has to get over the idea that the private sector doesn’t create any jobs,” but that he’d rather have Warren “with a couple modifications” as president than Trump.