To be more precise, should the tort of fraudulent misrepresentation apply to promises made by politicians running for office? (Put another way, why are most claims of fraud or deceit limited to commercial cases?) One reason politicians are generally not liable for fraud when they make false promises is the common law doctrine of “justified reliance.” In other words, to win a common law fraud case, the allegedly fraudulent statement must not only be material and factual; the plaintiff’s reliance on such statement must also be justified (whatever that means). Since representations of opinion, representations about law, and representations about the future are usually not considered material or factual, most politicians are off the legal hook, so to speak.
So which candidate lied the least?
prior probability 
Obama lied the least. But we can never really determine the extent of Mitt’s lies since he didn’t win. And yes they should be liable for their false advertising.
Politicians, just as any other individual, should not be held liable for their comments and proposed goals for the future. Everyone would be in court, paying fines and doing jail time if liability for false statements was enforced. Just because a politician is a public figure and promising to help the public if elected does not mean they should be sued if and when they fail to implement the campaign ideals they proposed. However, I do believe if a politician makes a specific platform their main campaign goal and once they are in office they completely have a change of heart and act as if they never endorsed the platform they should be liable. I do not know if they should liable through legal action because that would be difficult to enforce, but they should be liable in some way in this extreme case.
Definitively, a politician should be subject to liability. It is common in politics (specially on campaign seasons) watch constituents relying on politician promises to exercise their vote. Is a trust issue.. I believe that every person should respect others and make a promise that you know is not real or achievable is a big offense for someone that intent to represent people.
In addition, if we punish this kind of conduct, the politicians would take in consideration their intentions and platforms. But, realistically the implementation of this liability in the political arena will be too complex.