The presidential candidacy of Bernie Sanders, a senator from Vermont and self-described socialist who will most likely champion the liberal cause, won’t change that fact that Hillary Rodham Clinton is poised to win the Democratic nomination without a serious contest.

That’s true even though the Democratic Party’s liberal activist base, which strongly opposed her bid in 2008, has considerable reservations about her ties to Wall Street, her foreign policy, the recent allegations about foreign donations to the Clinton Foundation and the revelations about the private email account and server she used when she was the secretary of state.

This is mainly because of Mr. Sanders’s own weaknesses as a candidate and Mrs. Clinton’s strengths. But there is another, strangely simple reason Mrs. Clinton will have an easy road to the nomination: The left wing of the Democratic Party just isn’t big enough to support a challenge to the left of a mainstream liberal Democrat like Mrs. Clinton.