WASHINGTON — The last time the nation’s most potent pro-Israel lobbying group lost a major showdown with the White House was when President Ronald Reagan agreed to sell Awacs surveillance planes to Saudi Arabia over the group’s bitter objections.

Since then, the group, the American Israel Public Affairs Committee, has run up an impressive record of legislative victories in its quest to rally American support for Israel, using a robust network of grass-roots supporters and a rich donor base to push a raft of bills through Congress. Typically, they pass by unanimous votes.

But now Aipac, as the group is known, once again finds itself in a very public standoff with the White House. Its top priority, a Senate bill to impose new sanctions on Iran, has stalled after stiff resistance from President Obama, and in what amounts to a tacit retreat, Aipac has stopped pressuring Senate Democrats to vote for the bill.

Officials at the group insist it never called for an immediate vote and say the legislation may yet pass if Mr. Obama’s effort to negotiate a nuclear agreement with Iran fails or if Iran reneges on its interim deal with the West. But for the moment, Mr. Obama has successfully made the case that passing new sanctions against Tehran now could scuttle the nuclear talks and put America on the road to another war.