Voters would vote against candidates because they had so much money behind them. 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.