I have always maintained that the Arab lobby is more influential than any other lobby in the US. This is a good article about it in the ME Quarterly, with some very interesting observations
Walt and Mearsheimer cavalierly dismissed the possibility that U.S. policy might be subject to countervailing influences by those who believe the national interest is best served by distancing the United States from Israel and cultivating ties with the Arab states. They are not alone. Many analysts have ignored or belittled the notion that an Arab lobby exists or has any influence.[5] Yet one need only look at the first year of the Obama administration to reject Walt and Mearsheimer's case. How can Obama's solicitous policy toward the Arabs and hostility toward Israel be understood if the Israel lobby is so omnipotent or if pro-Arab forces are nonexistent? While The Israel Lobby came out before Obama took office, one could as easily look to the hostility displayed toward Israel by the Eisenhower administration after the 1956 Suez War to see the fallacy of the hypothesis.
The term "Arab lobby" may be somewhat misleading because it suggests that the principal members are Arabs and that their focus is on the Arab world, but Arab Americans are only a small and mostly ineffective part of the overall lobby. Moreover, while one might think that the Arab lobby would reflect the interests of the various Arab states and the Palestinians, it has historically shown little sustained interest in other Arab countries or issues within those countries. The lobby does not campaign for human rights or better governance in any of these countries; does not defend women, Christians, or other minorities in Arab states; and does not even try to get aid for Arab governments. The only time any interest is shown in an Arab country is if Israel is involved as was the case in the recent Israel-Lebanon war when the lobby expressed great concern for the people of Lebanon. Prior to the event, the lobby never talked about issues such as the Syrian occupation, Hezbollah's takeover of the organs of government, the undermining of democracy, or the massacres perpetrated by Lebanese factions against each other.
One of the most important distinguishing characteristics of the Arab lobby is that it has no grassroots support. While the Israel lobby has hundreds of thousands of active members, and U.S. public opinion polls consistently reveal a huge gap between support for Israel and the Arab nations or the Palestinians, the Arab lobby has almost no foot soldiers or public sympathy.
The most powerful part of the Arab lobby is represented almost exclusively by Saudi Arabia and the corporate—especially oil companies—and diplomatic interests that view Saudi well-being as paramount to U.S economic and security concerns. No other Arab state has any representatives with even marginal clout; in fact, the American Israel Public Affairs Committee (AIPAC) often is the most effective lobbyist for Israel's diplomatic partners Jordan and Egypt, supporting both governments' requests for aid and at least tacitly approving U.S. arms transfers.
