WASHINGTON — President Obama had a tough message for the American Israel Public Affairs Committee, or Aipac, the powerful pro-Israel group that is furiously campaigning against the Iran nuclear accord, when he met with two of its leaders at the White House this week. The president accused Aipac of spending millions of dollars in advertising against the deal and spreading false claims about it, people in the meeting recalled.

So Mr. Obama told the Aipac leaders that he intended to hit back hard.

The next day in a speech at American University, Mr. Obama denounced the deal’s opponents as “lobbyists” doling out millions of dollars to trumpet the same hawkish rhetoric that had led the United States into war with Iraq. The president never mentioned Aipac by name, but his target was unmistakable.

The remarks reflected an unusually sharp rupture between a sitting American president and the most potent pro-Israel lobbying group, which was founded in 1951 a few years after the birth of Israel.

Ronald Reagan opposed Aipac when he defied Israeli objections over the sale of Awacs reconnaissance planes to Saudi Arabia in 1981. A decade later, George H. W. Bush took on the group during a fight over housing loan guarantees for Israel, saying he was just “one lonely little guy” going up against a thousand lobbyists on Capitol Hill.