The American-Israeli Public Affairs Committee (AIPAC) is one of the most powerful lobbies in America. They describe themselves as "America's Pro-Israel lobby". Although they are not registered as a Political Action Committee, and so can't officially give money directly to candidates, they fund and strongly influence the actions of more than a hundred pro-Israel PACs which they have set up, and which do give large sums. This gets over the limitation that a single PAC can only give a maximum of $2,000 to a single candidate during an election cycle. Despite this indirectness, it still makes sense for the money distributed by these pro-Israel PACs to be described as 'AIPAC money'.
The primary aim of AIPAC is ensuring that all legislation passed through the U.S. Congress is beneficial to Israel, and that no legislation passes which is beneficial to its enemies (eg the Palestinians, Iran, other Arab countries). The types of legislation include ensuring lucrative financial aid, military weapons, and loan forgiveness to Israel, as well as positive statements showing that the U.S. supports Israeli policies and decries Palestinian actions and policies. AIPAC has also been a powerful force in crafting U.S. foreign policy to push for wars against the enemies of Israel.
One of the major concerns is that AIPAC and its supporters are so powerful that their influence places the interests of Israel above the needs of Americans, sometimes undermining the national security interests of America. Nearly every politician running for office has chosen to accept AIPAC money in return for support from the pro-Israel faction, understanding it can be the kiss of death not to have the support of AIPAC. This means that potentially every politician who accepts money from AIPAC is beholden to AIPAC's influence. One of the more dangerous characteristics of AIPAC is the ability it has had to stifle debate on middle east policy, Israeli policies, and even on the power of AIPAC itself.
One of the first prominent Americans to write about AIPAC was Rep. Paul Findley (R-IL), a congressman for 22 years who initially knew little about the issues in the middle east. His book, They Dare to Speak Out (originally published 1989 and re-published several times, most recently in 2003), is a detailed examination and revelation about the impact of AIPAC on middle east policy and politics in America. Rep. Findley became involved with the issue quite by accident, and ended up trying to become a genuine peacemaker on behalf of Israel and Palestinians, meeting with Yasser Arafat at times. That's when Rep. Findley began to feel the impact of the lobby, and he attributes his eventual defeat from his seat in Congress to the aggressive tactics and influence of AIPAC.