WASHINGTON — On the eve of a crucial visit to the White House by Prime Minister Benjamin Netanyahu of Israel, that country’s most powerful American advocates are mounting an extraordinary public campaign to pressure President Obama into hardening American policy toward Iran over its nuclear program.

From the corridors of Congress to a gathering of nearly 14,000 American Jews and other supporters of Israel here this weekend, Mr. Obama is being buffeted by demands that the United States be more aggressive toward Iran and more forthright in supporting Israel in its own confrontation with Tehran.

While defenders of Israel rally every year at the meeting of the pro-Israel lobbying group, the American Israel Public Affairs Committee, this year’s gathering has been supercharged by a convergence of election-year politics, a deepening nuclear showdown and the often-fraught relationship between the president and the Israeli prime minister.

Mr. Obama and Mr. Netanyahu will both speak to the group, known as Aipac, as will the three leading Republican presidential candidates, who will appear via satellite from the campaign trail on the morning of Super Tuesday. Republicans have seized on Iran’s nuclear ambitions to accuse Mr. Obama of being weak in backing a staunch ally and in confronting a bitter foe.