AIPAC maintains a virtual stranglehold over the US Congress. Critics of the Israel lobby other than Walt and Mearsheimer also contend that AIPAC essentially prevents any possibility of open debate on US policy towards Israel.
AIPAC should not be crossed. It rewards those who support its agenda, and punishes those who don't. In the end, it's all about money - specifically campaign contributions. From 2000 to 2004, according to the Washington Post, AIPAC honchos contributed an average of $72,000 each to campaigns and political committees. For pro-AIPAC politicians, money simply pours from all over the US.
Every member of the US Congress receives AIPAC's bi-weekly newsletter, the Near East Report. Walt and Mearsheimer stress that Congressmen and their staff "usually turn to AIPAC when they need info; AIPAC is called upon to draft speeches, work on legislation, advise on tactics, research, collect co-sponsors and marshal votes".