WASHINGTON — With dark warnings and a call to action, Prime Minister Benjamin Netanyahu of Israel used one of the world’s most prominent venues on Tuesday to denounce what he called a “bad deal” being negotiated with Iran and to mount an audacious challenge to President Obama.

In an extraordinary spectacle pitting the leaders of two close allies against each other, Mr. Netanyahu took the rostrum in the historic chamber of the House of Representatives to tell a joint meeting of Congress that instead of stopping Iran from obtaining nuclear weapons, Mr. Obama’s diplomatic initiative “would all but guarantee” that it does, in turn setting off a regional arms race.

“This deal won’t be a farewell to arms,” Mr. Netanyahu told the lawmakers, who responded to him with a succession of standing ovations. “It would be a farewell to arms control. And the Middle East would soon be crisscrossed by nuclear tripwires. A region where small skirmishes can trigger big wars would turn into a nuclear tinderbox.”

Such dire predictions could make it much harder for Mr. Obama to sell an agreement to a Republican-led Congress even if his negotiators reach one in Geneva. The president quickly tried to counter the prime minister by dismissing the speech as “theater” and “nothing new.” Mr. Netanyahu, the president told reporters, had no better ideas than the status quo or, in theory, military strikes against Iranian facilities.