Sen. Claire McCaskill said Thursday she won't vote for Senate Majority Leader Harry Reid to stay on as the Democrat's top leader. Pictured, Reid speaks to reporters after the Democratic party policy luncheon in the Capitol in Washington, Sept. 16, 2014. Reuters

Nevada U.S. Sen. Harry Reid is poised to become the Senate minority leader, but he won't have Sen. Claire McCaskill's vote. The Missouri Democrat said Thursday she wouldn't back Reid only a week after Republicans seized control of Congress, demoting Reid from his Senate majority leader post.

“I will not,” McCaskill said when asked whether she would vote for Reid as she walked into the Old Senate Chamber, where Democrats are voting on their leadership this week. McCaskill, a frequent Reid critic over the years, ignored questions from reporters about why she wouldn't vote for him, according to Politico.

McCaskill is the first Democrat to challenge Reid's long reign over the Senate Democrats, according to media reports. She later issued a statement: “Yesterday I met with Harry Reid and told him I would not be supporting him for minority leader. I heard the voters of Missouri loud and clear. They want change in Washington. Common sense tells me that begins with changes in leadership.”

Other Democrats have hinted that they could vote against Reid, including Sens. Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota and Jeanne Shaheen of New Hampshire. No Democrats are campaigning to take his post.

Reid is expected to retain his majority leader title until the new Congress convenes in January and Senate Minority Leader Mitch McConnell takes control of the Senate. Upon formally reconvening the Senate Wednesday, he said he had long struck compromises with GOP leaders to pass policy. "Regardless of how you interpret last week's election results, it's clear that the American people want us to work together," he said, according to the Washington Post.

Reid previously served as Senate minority leader under former President George W. Bush.