* "If Israel is attacked, we have their back" - Romney
* "I will stand with Israel if they are attacked" - Obama
By Tabassum Zakaria
WASHINGTON, Oct 23 (Reuters) - U.S. President Barack Obama and Republican challenger Mitt Romney vied on Monday over who was Israel's strongest defender but both agreed that a military strike over Iran's nuclear program must be a "last resort."
Tehran's nuclear program, which the West suspects is for developing weapons and that economic sanctions have so far failed to stop, is almost certain to be among the top foreign policy challenges facing the next president.
Yet Romney and Obama, in their foreign policy debate, did not offer sharply contrasting policies to address the challenge. They agreed on the need for tough economic pressure - and for safeguarding U.S. ally Israel.
"If Israel is attacked, we have their back, not just diplomatically, not just culturally, but militarily," Romney said.
"I will stand with Israel if they are attacked," Obama said.
Iran's leaders have from time to time threatened to eradicate Israel, and Israeli leaders see an Iranian nuclear weapon as an existential threat. Israel has its own undeclared nuclear arsenal.
The question that has risen repeatedly this year is whether Israel would conduct a unilateral strike against Iran's nuclear sites, which would put the United States in a difficult position of whether to enter another Middle East conflict.
The United States and its allies have significantly ratcheted up sanctions against Iran. Obama has often said all options are on the table, but has counseled that diplomacy and sanctions must have a chance to work.
The candidates did not say what they would do if Israel conducted a unilateral strike on Iran, and at one point Romney brushed aside a hypothetical question on what he would do if the Israeli prime minister called to inform him Israel's bombers were en route to strike Iran's nuclear facilities.
"The disagreement I have with Governor Romney is that, during the course of this campaign, he's often talked as if we should take premature military action," Obama said at the final debate before the Nov. 6 election.
"I think that would be a mistake, because when I've sent young men and women into harm's way, I always understand that that is the last resort, not the first resort," he said.
Romney and Obama both said it was important to prevent Iran from obtaining a nuclear weapon.
"We need to increase pressure, time and time again, on Iran because anything other than ... a solution to this ... which stops this, this nuclear folly of theirs, is unacceptable to America," Romney said.
"And of course, a military action is the last resort. It is something one would only ... consider if all of the other avenues had been ... tried to their full extent," he said.
Romney pressed his campaign's argument that Obama has been an insufficient friend to Israel, and criticized the president for not visiting the country since he has been in the White House.
Clearly annoyed, Obama criticized Romney for taking fundraisers on a summer trip to Israel and said that on his own trip as a presidential candidate, he visited the Israeli city of Sderot, a frequent target of missiles launched from the Gaza Strip by the militant group Hamas.
The result, he said, was his administration's funding support for an Israeli missile defense system called Iron Dome.
Rick "Ozzie" Nelson, an international security expert at the Center for Strategic and International Studies, said the foreign policy debate was "underwhelming" and the candidates kept switching to domestic policies.
"I thought they both were saying the same thing on Iran. Their policies didn't differ very much," Nelson said. "I didn't see anything different about their policy in Iran, particularly their policy regarding Israel as well. They were in sync on that."
Obama bluntly said newspaper reports that Iran and the United States had agreed to hold bilateral talks on Tehran's nuclear program were not true.
Iran has also denied that bilateral negotiations on its nuclear program had been scheduled.
The United States and other Western powers say Iran's nuclear program is aimed at developing nuclear weapons, but Tehran says the purpose is purely peaceful.
On Syria, Romney tried to put Obama on the defensive by saying the administration has not led in the crisis in which thousands of Syrians have died and President Bashar al-Assad remains in power.
"What I'm afraid of is, we've watched over the past year or so, first the president saying, well, we'll let the U.N. deal with it," Romney said. "We should be playing the leadership role there."
The United States should work with partners to organize the Syrian opposition and "make sure they have the arms necessary to defend themselves," he said.
"I am confident that Assad's days are numbered," Obama said. "But what we can't do is to simply suggest that, as Governor Romney at times has suggested, that giving heavy weapons, for example, to the Syrian opposition is a simple proposition that would lead us to be safer over the long term."