Christopher Hitchens writes:

To take an example near to hand: A few months ago, I wrote here that the recent sharp deterioration in Israeli-Turkish relations was at least partially explicable by a single fact: This year, a key House committee voted to refer to the Turkish massacre of the Armenians in 1915 as genocide. In previous years, that vote had gone the other way. The difference, I pointed out, was this: Until recently, the Israel lobby on the Hill had worked to protect Turkey from such condemnation. But after the public quarrel between Turkey's prime minister and Israel's president at Davos, the lobby was in no mood to do any more favors. In other words, a vote with major implications for U.S. foreign policy—positive ones in my opinion—was determined by the supporters of a single power. I did not receive a single letter of complaint for making this observation, and I know nobody in Washington who would have quarreled with its obviousness.

A single power? Really? In the same New York Times story that Hitchens links to:

The vote on the nonbinding resolution, a perennial point of friction addressing a dark, century-old chapter of Turkish history, was 23 to 22. A similar resolution passed by a slightly wider margin in 2007, but the Bush administration, fearful of losing Turkish cooperation over Iraq, lobbied forcefully to keep it from reaching the House floor. Whether this resolution will reach a floor vote remains unclear.

The resolution, with the same wording, has been reported favorably by the same committee four times since 2000: 24-11 in 2000, 40-7 in 2005, 27-21 in 2007, and 23-22 in 2009

As for the Armenian lobby's power in Washington, here's Zbigniew Brzezinski in Foreign Policy:

The participation of ethnic or foreign-supported lobbies in the American policy process is nothing new. In my public life, I have dealt with a number of them. I would rank the Israeli-American, Cuban-American, and Armenian-American lobbies as the most effective in their assertiveness.

Der Spiegel, in an article about the 2007 resolution entitled “The Triumph of the Armenian Lobby”:

Armenian-Americans are particularly active in California, New Jersey and Michigan -- which happens to be the constituency of Nancy Pelosi, the Democratic Speaker of the House. Her Californian colleague Adam Schiff, who promoted the resolution, has the issue to thank for his own political career. His predecessor in the constituency lost his seat when he failed to push through the resolution in 2000. Armenian groups have been bombarding their representatives over the past few years with an unusually massive PR drive. Their most important umbrella group "Armenian Assembly of America" has 10,000 members and an annual budget of over $3.5 million. It employs four different influential PR firms in Washington to keep the suffering of the Armenians on the agenda in the US capital. The Turkish government couldn’t do enough to counter them, even though for years it has invested millions of dollars in presenting its arguments.

Radio Free Europe:

There are lobbying groups representing the concerns of nearly every diaspora community in the United States. The Armenian lobby is one of the most successful.

The BBC:

Given that Armenians represent only about 1.5m of America's 300m population, what has won them such influence over the US Congress - and perhaps the nation's foreign policy? Part of the answer lies in the organisation and determination of the Armenian-American lobby groups, says Dr Svante Cornell, of the Central Asia-Caucasus Institute at Johns Hopkins University… "The Turkish lobby is much less organised and much less rooted in an electorate than the Armenian lobby,” [Dr. Cornell said.]

Nobody denies that the Israel lobby is powerful. That is a straw man. The issue at stake is the habit of Israel's critics of attributing almost unique powers to the Israel lobby. It is not merely one lobby, or even the first among equals, but a sinister force whose powers stand above all others -- The Lobby, as Walt and Mearsheimer called it.

The truth is that the Israel lobby is a lobby, not the lobby. It influences American policy but it does not control it.