McCaskill: Lindsey Graham 'has lost his mind'

Former Sen. Claire McCaskillClaire Conner McCaskillBig Dem names show little interest in Senate Gillibrand, Grassley reintroduce campus sexual assault bill Endorsements? Biden can't count on a flood from the Senate MORE (D-Mo.) remarked Wednesday that Sen. Lindsey GrahamLindsey Olin GrahamTrump: Anonymous news sources are 'bulls---' Trump: 'Good chance' Dems give immigration 'win' after Pelosi called White House plan 'dead on arrival' The Hill's Morning Report — Presented by Pass USMCA Coalition — Trump: GOP has `clear contrast' with Dems on immigration 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 John TrumpComey: Barr is 'sliming his own department' GOP Mueller critic says Flynn contacted him during special counsel probe: report Acting DHS secretary threatened to quit after clashing with Miller: report 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 McCainSeveral factors have hindered 'next up' presidential candidates in recent years Small Florida county that backed Trump one of two targeted by Russians: reports Howard Stern defends McCain from Behar on 'The View': 'Give the woman a chance!' 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.