Tuesday’s midterms served up mixed results, handing both parties big wins and big losses. It will take some time to sort out what it means. Yet the evening did nonetheless provide one total, complete, unalloyed loser: Massachusetts Sen. Elizabeth Warren.

For a decade Ms. Warren, 69, has been busy trying to remake Washington in her progressive image. Her role in creating a new financial regulatory apparatus gave her outsize influence over the bureaucracy. Her successful 2012 Senate bid gave her a megaphone to rail against “billionaires,...