It's time for Hillary Clinton to give up her futile quest for the U.S. presidency. She needs to get out soon, putting a united Democratic party ahead of personal ambition before her reputation in a sometimes-ugly campaign is further sullied. After Barack Obama's blowout victory in Wisconsin last week, the Clinton camp vowed to "go negative" on an opponent who has won more than twice as many primary-season contests as Clinton; leads her in popular vote and pledged delegates; is making gains in Clinton's base of women, blue-collar workers and Hispanics; and is shown in most polls beating John McCain,...