Sen. Elizabeth Warren admitted last Friday that she had made a colossal, potentially fatal, campaign error—and immediately proceeded to make it worse. If the Warren presidential bid flops, this will be the moment to mark.

That admission didn’t come in so many words. It came instead in the form of a major update to Ms. Warren’s Medicare for All plan. The Massachusetts senator now proposes a two-year “transition” period, in which Americans would be able to opt in to Medicare. Put another way, Ms. Warren now calls for the same...