【正确答案】The single greatest weakness of American parties is their inability to achieve cohesion in the legislature. Although there is some measure of party unity; it is not uncommon for the majority party to be unable to implement important legislation. The unity is strongest during election campaigns; after the primary elections, the losing candidates all promise their support to the party nominee. By tile time the Congress convenes, the unity has dissipated. This phenomenon is attributable to the fragmented nature of party polities. The national committees are no mote than feudal lords who receive nominal fealty from their vassals. A congressman builds his own power upon a local base. Consequently, a congressman is likely to be responsive to local special interest groups. Evidence of this is seen in the differences in voting patterns between the upper and lower houses. In the Senate, where terms are longer, there is more unity.