Why Did Warren's Attacks On Sanders Backfired?

Three weeks ago, Elizabeth Warren and Bernie Sanders were locked in a vicious battle.
Warren said Sanders told her that a woman couldn't be elected President.
Sanders denied making the statement.
Warren said Sanders had sent teams of staffers with a script to bad mouth her to voters.
Sanders denied this. But, later, he admitted his team did use a script to attack Warren.
Warren confronted Sanders during the January debates.