McCaskill: Lindsey Graham 'has lost his mind'

Former Sen. Claire McCaskillClaire Conner McCaskillLobbying world Dem candidate has Hawley served subpoena at CPAC Annual scorecard ranks GOP environmental efforts far below Dems in 2018 MORE (D-Mo.) remarked Wednesday that Sen. Lindsey GrahamLindsey Olin GrahamDem senator: 'Appropriate' for Barr, Mueller to testify publicly about Russia probe Conservatives wage assault on Mueller report Graham expects 'thorough' briefing on Mueller report 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 TrumpMueller report findings could be a 'good day' for Trump, Dem senator says Trump officials heading to China for trade talks next week Showdown looms over Mueller 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 McCainFormer astronaut running for Senate in Arizona returns money from paid speech in UAE Fox's Roberts: Trump 'glared at me like I've never seen him glare at me before' Lou Dobbs: Political criticism of McCain 'not an exhumation of his body' 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.