Bernie Sanders is this election's Democratic sheepdog. The sheepdog is a
card the Democratic party plays every presidential primary season when
there's no White House Democrat running for re-election. The sheepdog is
a presidential candidate running ostensibly to the left of the
establishment Democrat to whom the billionaires will award the
nomination. Sheepdogs are herders, and the sheepdog candidate is charged
with herding activists and voters back into the Democratic fold who
might otherwise drift leftward and outside of the Democratic party,
either staying home or trying to build something outside the two party
box.
-- Bruce Dixon, "Presidential Candidate Bernie Sanders: Sheepdogging for Hillary and the Democrats in 2016" (Black Agenda Report).