Washington, Oct. 17 -- Elizabeth Warren is rising to the top of the Democratic pack with ambitious promises to reshape the political and economic system. But as she faces growing scrutiny, the Massachusetts senator is opening herself to criticism that she's just another politician dodging the tough questions.

She is in a bind because of her persistent refusal during two straight presidential debates to say whether she would raise taxes on the middle class to pay for the universal health insurance plan known as "Medicare for All."

By not acknowledging taxes would almost certainly increase for a wide range of income earners, Warren avoids becoming a caricature of a Democrat itching to raise them. But she also threatens to undermine the im...