Liberals will typically do whatever it takes to promote their agenda regardless of principles or double standards, and then they try to build the justifications for what they did afterwards. A perfect example of this is Obama promising to use the public campaign finance system until he saw that it was to his advantage to break the promise. Having broken his promise, he offered some mealy-mouthed justification by saying that the system needed fixing.
Obama’s broken promise regarding public campaign finance was so blatant that even some in the MSM commented negatively on it, but then they quickly moved on. It would have received at least 20 times the negative coverage if McCain had done the same thing.