Look at the criticism Ed Milliband got for deciding that he couldn't support an attack on Syria as he had been expected to do. If he hadn't done so, we might have ended up being dragged into an attack with the USA, but here we are with at least some hope of an advance through diplomacy with the Russians and the Chinese taking part. So much for "letting down the Americans" as he was accused of doing. More like "giving the Americans a bit of breathing space to see if something else can be worked out". If he was swung by argument or even just by giving it a bit of thought, he deserves much credit, IMO.
I say this as one who does not vote Labour (or Tory, come to that).