What forces the policy maker to obey the law

When law directs a policy maker to carryout policy, what forces the policy maker to obey the law? When there is no restriction from law, the policy maker will be force to change or revise the policy to promise to benefit his constituency with a view of winning next election or to raise his percentage of voters. If this law is passed and directs the policy maker to carry out the policy, again he will be tempted to break the law to benefit his constituency in the name of increase in the social welfare. Further there are policy promises, which have no legal backing as well. In fact it is the penalty associated to every law that conditions and restricts the behaviour of the agents. It can enhance credibility by raising the cost and lowering the benefit when one deviates from a given policy.