The concept of one great national party dominating the lower echelons does not exist in America. The heartbeat of party politics does not come form Washington DC and therefore the idea of a national party is difficult to consider for America. Members of Congress are well aware that their electoral success is due to those who voted them into office. Therefore, the pressure party whips can bring onto these politicians is weak. In Britain, the opposite is true.
Party discipline is usually very strong.
The power of the Chief Whip is immense though frequently unseen. In Britain, any promotion within the party, comes from the party itself. To advance yourself, you have to please the party leadership and, therefore, any movement away from the central party line will probably result in a politician not being promoted. There have been instances of politicians going against the party line, but these people are frequently seen as mavericks condemned to a life on the back benches. It would be wrong to state that party discipline is non-existent in Congress.