Mr. Blunt is now running as if Mr. Kander were the incumbent. He ties the Democrat to cost increases in the “nightmare” Affordable Care Act, to “out of control” federal regulators and to the liberal agenda of Mrs. Clinton, who remains highly unpopular among Missourians.

Mr. Kander, hoping to tap into some of the anti-Washington Trump vote, counters that a “new generation” of politicians is needed to undo the special-interest establishment epitomized by Mr. Blunt, a 20-year veteran of the House and Senate. Mr. Kander pointedly notes that Mr. Blunt’s wife and three adult children all prosper as registered lobbyists, to which Mr. Blunt tepidly replies, “Everybody’s family does something.”

Though Mr. Blunt has acted in a bipartisan manner on some issues, he has largely toed the party line, including the Republicans’ irresponsible unwillingness to hold hearings on Judge Merrick Garland, President Obama’s nominee for the Supreme Court.

With political cynicism like that, Mr. Blunt is effectively making his opponent’s case that Missourians do indeed need fresh blood in Washington. Mr. Kander displays clear energy for change on a raft of policies, from improving international trade treaties to redefining presidential war powers too outdated for threats like the Islamic State. Mr. Blunt basically hangs his chances on Mrs. Clinton’s unpopularity in the state, insisting Missourians will be better off with a President Trump.