JERUSALEM (Reuters) -- US Republican presidential candidate Mitt Romney's campaign left a troubled visit to London behind him on Saturday and looked to rebound in Israel, whose ties with the United States have been a familiar theme during his election bid.

Romney arrived late on Saturday on the second leg of a foreign trip aimed at bolstering his foreign-policy credentials in his race to unseat Democratic President Barack Obama in the Nov. 6 election.

The trip has been beset with difficulties from the start, when he raised British hackles by questioning whether London was ready for the Summer Olympics, a statement he was forced to walk back after a rebuke from Prime Minister David Cameron.

The former Massachusetts governor hoped to find fairer sailing winds in Israel by returning to a familiar issue in his campaign, pledging stronger ties between the United States and Israel if he is elected.

He and conservative Israeli Prime Minister Benjamin Netanyahu are old friends, and they will meet on Sunday before Romney gives a speech.

His visit coincides with a Jewish fast day that commemorates tragedies that have befallen the Jewish people.

Romney carried with him a tough message on Iran's nuclear program with senior national security aide Dan Senor saying the presidential candidate believes the threat of Tehran developing a weapon should be handled aggressively.

"He believes there should be zero tolerance for uranium enrichment as it relates to Iran. He believes that the threat of military action has to be credible in the eyes of the Iranian leadership," he said.

"The only kind of deal (with Iran) that is acceptable to him involves zero uranium enrichment. And we can and should be deploying tough sanctions. There are tough sanctions in place. We can put in place tougher sanctions," Senor said.

As for the possibility of a unilateral Israeli strike on Iran's nuclear facilities, which the Obama administration has sought to avoid, Senor told reporters on Romney's plane that the candidate believes Iran developing a nuclear weapons capability is an existential threat to Israel and a threat to the United States, as well.

"Like-minded countries should work together, partner together in addressing that threat head on. He's always said if Israel believe it needs to take measures to defend its country, protect its country, he would be supportive," the aide said.

Fundraiser

While Romney will give a high-profile speech on Sunday, he ends his trip on Monday taking in cash from a crowd of mostly Jewish Americans who live in Israel.

But the fundraiser, scheduled for a Jerusalem hotel, was declared off limits to the news media by the Romney campaign. Officials gave no explanation for the decision, but keeping the press at bay would conceivably allow him to avoid distractions.

The campaign had kept finance events closed to the press for months, but in May began allowing a pool of reporters into fund-raisers that are at public venues like hotels, while keeping closed those at private residences.

The event is notable in that one of the attendees is expected to be Sheldon Adelson, a Las Vegas billionaire who had helped bankroll a political action committee in support of Newt Gingrich, a Romney opponent during the Republican primary battle.

Adelson, a strong supporter of Netanyahu and eager to defeat Obama, has sent more than $10 million to a similar group operating on Romney's behalf.

Romney's visit to Israel carries political sensitivities for him as he seeks to carefully navigate the complicated world of Middle East politics without violating his personal vow not to criticize Obama while on foreign soil.

He argues that Obama has loosened the normally tight bonds between the United States and Israel by proposing in 2011 a peace deal between Israel and the Palestinians that would return Israel to pre-1967 borders.

Romney will meet Palestinian Authority Prime Minister Salam Fayyad, but his visit is centered mostly on Israeli leaders.