There are at least three ways which lead a representative or a senator to vote for or against a bill or amendment: representational, organizational and attitudinal. Representational is based on the assumption that members want to get re-elected and therefore vote to please their constituents. Organizational is based on the assumption that since most constituents don't know how their senator voted, it is not essential to please them. But it is important to please fellow members of Congress. The attitudinal is based on the assumption that there are so many conflicting pressures on members of Congress that they cancel one another out, leaving them virtually free to vote on the basis of their own beliefs.
The representational view has merit under special circumstances, like when the constituents have a clear view on some issue and a legislator's vote on that issue will draw much attention. For instance, a legislator from a highly black district won't vote against a civil rights bill for minorities, while representatives with mostly non-minority (white) voters in their district can vote whichever way they please on the issue.
Foreign policy is generally remote from the daily interest of most Americans and the public changes opinions on it rather rapidly. The constituents and the legislator vote differently for the most part on foreign policy. When an issue arouses deep passion among voters, it is necessary for the legislator to go in accordance with the majority constituents, despite his or her personal convictions if he or she wants to be re-elected. The general problem with representational explanation is that public opinion is not strong and clear on most measures on which the Congress votes. Many representatives and senators face constituencies that are divided on key issues.
The organizational view deals more with legislators voting in accordance to...