JERUSALEM—Mitt Romney is planning to thrust his campaign into an intensifying debate over stopping Iran’s pursuit of nuclear weapons, saying he would support a unilateral Israeli military strike on Tehran, according to an aide who briefed reporters on its contents.

In a foreign policy speech that he is scheduled to deliver on Sunday night, Romney will stress the importance of the US-Israel relationship, but also highlight its right to wage an attack on Iran.

“It is an existential threat and we in the West partnering with Israel should do everything we can from stopping Iran from developing that weapons capability,” Dan Senor, a top Romney foreign adviser who helped organize the trip to Israel. “And if Israel has to take action on its own, in order to stop Iran from developing that capability the governor would respect that decision.”

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Romney has said that US military action should be “on the table,” but he has cautioned that it should be a last resort. Still, the rhetoric that he would support an Israeli strike on Iran puts Romney at odds with President Obama, who has urged Israel to give diplomacy a chance.

Romney’s campaign later softened his stance.

“Governor Romney believes we should employ any and all measures to dissuade the Iranian regime from its nuclear course, and it is his fervent hope that diplomatic and economic measures will do so,” Senor said in a statement about four hours after his briefing. “In the final analysis, of course, no option should be excluded. Governor Romney recognizes Israel’s right to defend itself, and that it is right for America to stand with it.”

Romney arrived in Israel on Saturday night, placing him in one of the world’s most volatile areas of conflict, and a place where every word is parsed for meaning. Romney spent much of Sunday morning in meetings, where discussions also shifted to Iran’s nuclear threat.

“We have to be honest and say that all the sanctions and diplomacy so far have not set back the Iranian program by one iota,” Israel Prime Minister Benjamin Netanyahu said before meeting with Romney. “And that’s why I believe that we need a strong and credible military threat coupled with the sanctions to have a chance of changing that situation.”

Netanyahu and Romney have a relationship that extends back to the 1970s, when they both worked at Boston Consulting Group, after Romney obtained graduate degrees from Harvard and Netanyahu from MIT.

“We’ve known each other for many decades, since you were a young man, but for some reason, you still look young.” Netanyahu said, as Romney laughed. “You’ve been a personal friend of mine and a strong friend of the state of Israel, and that’s why it’s a pleasure to welcome you here.”

Romney and his wife, Ann, later made an unexpected trip to the Western Wall, one of the holiest sites for Jews. Romney was escorted to the wall by a rabbi, as a crowd formed, snapping pictures with cell phones, moving to shake his hand, and shouting at him

“Beat Obama, governor! Beat Obama,” one man shouted. “Get rid of Obamacare!” another said.

That group that accompanied Romney down to the wall included some of Romney’s donors, who are in town for a high-dollar fundraiser on Monday morning. The campaign announced on Sunday that reporters would be allowed to cover Romney’s remarks at the fundraiser, reversing a decision to bar reporters in violation of a previously-arranged agreement with the media for covering such events.

Romney’s trip to Israel, which lasts only about 36 hours, allows him to put on a statesman’s hat and demonstrate to American voters at home that he has the skills to be commander in chief. It comes after an unsteady start to his trip in London, where a disparaging comment he made about the city’s Olympic Games attracted rebukes from British newspapers and politicians.

Romney has ruffled some feathers in Jerusalem as well. His campaign initially scheduled a meeting with leaders of the Labor Party, which until two weeks ago was the primary opposition party. But the campaign canceled the meeting about 90 minutes before it was to occur.

“We were disappointed because we wanted Governor Romney to hear our position,” Isaac Herzog, a leader of the Labor Party, who met Romney during his 2007 trip to Israel, said in an interview. “We were saddened by this, and quite surprised.”