willow in the wind

(idiomatic) One whose views are easily and regularly changed by the persuasion or influence of others.

Example: 1984 March 22, "Editorial: Smoking ban still up in the air," Reading Eagle (USA), p. 4 (retrieved 21 June 2011):
  [T]he CAB has rightfully seen Congress as a willow in the wind, bending to the wishes of whatever special interest groups and businesses speak loudest.
1989, Gerry Spence, With Justice for None, ISBN 9780812916966, p. 5:
  Justice is not a willow in the wind; justice stands immutable against unjust forces.
1992 June 25, "Local: Florida Senate," Miami Herald, p. 5B (retrieved 21 June 2011):
  If he has to vote against taxes, he will—and his vote won't change from day to day: “I'm not gonna be a willow in the wind.”
2008, Ronald Arthur Howard and Clinton D. Korver, Ethics for the Real World, ISBN 9781422121061, p. 40:
  Ethics begin to feel situational, a balancing of concerns. When this happens, we no longer have any firm ethic to stand on. We become an ethical willow in the wind.