The case of Perot shows that it is easy for a third-party candidate with serious mass support to enter at the highest level. Elections, unlike markets, are zero-sum games. Don't forget Tiebout-type competition. F. Third , empirical evidence shows a strong link between voter preferences and legislative behavior. Democracy is designed to have low transactions costs.
Majority rule is cheaper than the unanimity required by markets.
Instead of 250 M Americans bargaining, we have a few hundred Congressmen and Senators bargaining. Log-rolling can turn efficient but unpopular policies into efficient AND popular policies. Long-term political contracts are rarely legally enforceable. Interest groups also reduce transactions costs by giving legislators information. In essence, this means that some market actors know more than others about some crucial facts.