It is the area of the electoral college that concerns the constitution most. Article II, Section 1 of the U.S. Constitution mandates that the President be elected by means of an electoral college. Citizens for true democracy make a good point when they establish that we have already amended the constitution to allow for direct election of senators. In other words, we have already set legal precedent. The argument against the electoral college is a strong one.
Many states use a winner takes all strategy, and the individual vote is lost.
Then there is the power position the two main parties hold. Third parties hold little chance under the current electoral system. One example this site gives is Ross Perot. How exactly would we amend the electoral article of the constituion is questioned next, and Citizens for true democracy has an answer. It suggests such an amendment must be originated and ratified by the states, since federal powers that be have too much to lose here.