Professional groups representing doctors are buying into Democratic plans to remake health care—and thereby acting against the interests of their members. The American College of Physicians, the second-largest organization of U.S. doctors, recently came out in support of either a public option or single payer. At the American Medical Association’s annual meeting last year, some 47% of delegates voted to reverse the organization’s opposition to single payer. A new organization styling itself Physicians for a National Health Program has attracted some 20,000 members.

Some...