A cardinal rule of politics is don’t let your opponents define you. This has been a particular challenge for Sen. Elizabeth Warren, whose critics continue to insist that she is a left-wing radical. I am a Republican and have known and worked with Ms. Warren for many years. She is a capitalist and prairie populist, in the tradition of William Allen White and Teddy Roosevelt. She believes in a market economy. She just wants it to work for everyone.

Like me, she grew up in an economically depressed community in the rural Midwest—she’s...