President Obama’s regulators aren’t slowing down, alas. And on Wednesday they unveiled another part of their plan to push Americans out of private investment accounts and into government-run plans.

The Department of Labor says its so-called fiduciary rule will make financial advisers act in the best interests of clients. What Labor doesn’t say is that the rule carries such enormous potential legal liability and demands such a high standard of care that many advisers will shun non-affluent accounts. Middle-income investors...