The American Israel Public Affairs Committee (AIPAC), the most influential pro-Israel group in the U.S. and one of the most powerful lobbying organizations in Washington, told key leaders on a conference call Wednesday that it would fight the Iran deal “with the entirety of our institutional resources.” AIPAC had said the day before that it needed time to study the details of the 159-page Iran nuclear agreement.

Until now, AIPAC has avoided confrontations with the Obama administration. The group’s initial strategy when President Barack Obama took office was to cultivate personal ties with him–for example, by electing a Chicagoan to lead the group. It refrained from opposing former Sen. Chuck Hagel for Secretary of Defense in 2013, despite his poor record on Israel, and in 2014 it backed away from new sanctions on Iran.

Now, however, faced with an agreement that could allow the Iranian regime to become a nuclear power by cheating or merely waiting several years until the deal expires, AIPAC has decided to lobby Congress vigorously to vote against the deal and to override an expected veto by President Obama. AIPAC executive director Howard Kohr told the conference call: “This is the moment for which AIPAC was built.”

To that effect, AIPAC plans meetings in Washington later this month, and will be lobbying pro-Israel members of Congress–mostly on the Democratic side, since Republicans are expected to unite against the deal. Critics note that AIPAC missed earlier opportunities to oppose Obama’s policies on Iran, but AIPAC’s leadership carefully avoided burning political bridges so that it could reach out at a crucial time.