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McCaskill: Lindsey Graham 'has lost his mind'

Former Sen. Claire McCaskillClaire Conner McCaskillThe Hill's Morning Report - Biden tasks Harris on border; news conference today Missouri Senate candidate Eric Greitens tangles with Hugh Hewitt in testy interview The Hill's Morning Report - Presented by the National Shooting Sports Foundation - CDC news on gatherings a step toward normality MORE (D-Mo.) remarked Wednesday that Sen. Lindsey GrahamLindsey Olin GrahamMSNBC's Joy Reid pans Manchin, Sinema as the 'no progress caucus' Overnight Defense: Biden proposes 3B defense budget | Criticism comes in from left and right | Pentagon moves toward new screening for extremists Biden defense budget criticized by Republicans, progressives alike MORE (R-S.C.) is no longer "the man I served shoulder to shoulder with" in the upper chamber.

"Lindsey Graham has lost his mind. I don't even understand what has happened," McCaskill said on MSNBC, where she has served as a political analyst after leaving office last month.

The former Democratic lawmaker made the comment in reference to Graham's support for President TrumpDonald TrumpHarry Reid reacts to Boehner book excerpt: 'We didn't mince words' Man arrested for allegedly threatening to stab undercover Asian officer in NYC Trump says GOP will take White House in 2024 in prepared speech MORE's potential use of a national emergency declaration to build a wall along the southern border.

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McCaskill added on MSNBC that Graham, a staunch ally of Trump, is no longer "the man I served shoulder to shoulder with for so many years."

"I really don't understand. It must have something to do with how he views the president," she said.

Graham, a former 2016 GOP presidential rival to Trump, has supported Trump on many issues since he took office, but has also criticized the president over key moves, including Trump's abrupt decision in December to withdraw U.S. troops from Syria.

A number of prominent Republicans have recently urged Trump not to declare a national emergency to build a wall along the southern border, but Graham has said the GOP should get behind Trump on the issue.

McCaskill on Wednesday also pointed to Trump's recent criticism of the late Sen. John McCainJohn Sidney McCainColbert mocks Gaetz after Trump denies he asked for a pardon Five reasons why US faces chronic crisis at border Meghan McCain calls on Gaetz to resign MORE (R-Ariz.), who was a close friend of Graham's. Trump on Tuesday reportedly said McCain "wrote a book and the book bombed," apparently referring to McCain’s 2018 book, “The Restless Wave: Good Times, Just Causes, Great Fights, and Other Appreciations.”

McCaskill said she couldn't understand why Graham didn't throw a "haymaker in defense" of McCain.

“Frankly, the notion that this president was insulting John McCain yesterday and Lindsey Graham is standing tall for him, I do not understand why Lindsey Graham is not throwing a haymaker in defense of John McCain towards this president," she said.