She's a multimillionaire politician from the state where two rivers come together, to leave.
Gotta go. That state can and might replace her with a Republican, but she's one in soul and spirit so the only difference would be a vote on organizing. If she faces a progressive in a primary challenge, and it leads to a Republican win, there is no substantial loss. If she faces a progressive challenge leading to a progressive Senator replacing her, that would be bonanza time for certain.
The Senator, in two screencapture images, first from The Observer, here, second from Ballotpedia, here.
Last image, bottom, she's not Joe Manchin, but that is faint praise. Manchin is every progressive's lead "gotta go" federal officer/Democratic Party liability. Young people have cause to question a callous McCaskill position wanting them under the pressing stone of early and great debt loads. She should fade away. As a lesser evil than Todd Akin, whoopee. Taxing Wall Street trades could pay for free state college tuition for a deserving student body. In total, no strings attached.
Educated people are a national asset for a better fairer nation, which is the cause of Republican opposition to educating people. McCaskill appears to be in that camp. She's a big money constituency "Democrat." The kind that lost to Donald Trump. A Fade Away Democrat.
UPDATE: The Observer cross links two McCaskill online items, the one previously linked, and this.