Tuesday’s Democratic debate showed that Bernie Sanders still has some pep in his step, Joe Biden needs a better answer on his son Hunter’s business dealings, and Minnesota Senator Amy Klobuchar deserves more media respect. But the most important news was Senator Elizabeth Warren’s determined refusal to say if her plans would require taxes to increase on the middle class.

Ms. Warren, the new leader in the polls, was given at least six chances to answer yes or no. She ducked every time. “Will you raise taxes on the middle class...