Andrew Sullivan asks:

According to [Frank] Luntz,
56% of Americans agree with the claim that there is a humanitarian crisis in Gaza; and 43% of Americans agree with the claim that people in Gaza are starving.
More to the point, only 34% of Americans supported the Israeli operation against the Flotilla. That compares with 71 percent of members of Congress signing the AIPAC-backed resolution defending the raid. What accounts for this extraordinary discrepancy between the views of Americans and the views of the congressmen who allegedly represent them?

It's pretty simple. Americans are highly sympathetic to Israel in general:

In the Middle East situation, are your sympathies more with Israel or more with the Arab nations?
More with Israel .................... 61
More with the Arab nations ... 12

They have also been paying very little attention to the Flotilla incident:

Statement A: Israel was defending its interests in Gaza and their military actions were justified.
Statement B: Israel went too far and their military actions were NOT justified. 6/10**
Statement A: Defending interests/military actions were justified .............. 34
Statement B: Too far/military actions were NOT justified ......................... 29
Don’t know enough .................................................................................. 32

Congress is representing the generalized pro-Israel feelings of the public, which are strong, as opposed to their ambivalent views of the Flotilla incident, which are weak. In an environment where public opinion is overwhelmingly pro-Israel, and when asked about a specific incident involving Israel, two-thirds of the public is either favorable to Israel or uninformed, the expected outcome is for Congress to support Israel. Likewise, most Democrats like President Obama and dislike the Republican leadership in Congress. Democratic representatives in Congress are usually going to support Obama over the GOP even on issues where Democrats lack well-formed pro-Obama views, especially issues with low salience.

And, to be sure, even if there wasn't such a strong base of popular support for Israel, Congress is going to respond to those constituents have have the strongest views on the matter. That's why Congress sides strongly with Armenia over Turkey even though there's hardly a major groundswell of popular support on that issue. This is a routine facet of interest group politics.

Or maybe... I'm just saying this because I'm part of The Lobby.