Former Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) remarked Wednesday that Sen. Lindsey Graham Lindsey Olin GrahamGraham: GOP will confirm Trump's Supreme Court nominee before the election Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee 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 Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid 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 McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy 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.