WASHINGTON, July 21 (Steve Holland/Reuters) - Republican presidential candidate Mitt Romney will soon do something Barack Obama has yet to do as president - visit U.S. ally Israel, where he will try to present himself to voters back home as a credible replacement to Obama on the world stage.

In the midst of a presidential campaign that is too close to call, Romney leaves on Wednesday for a week-long trip to attend the opening ceremonies of the Olympics in London and visit Israel and Poland.

The aim is to give the former Massachusetts governor some foreign policy credentials and let Americans get a glimpse of him in action overseas as they decide whether to vote for him in the Nov. 6 general election to replace Obama.

Aides say the trip is a listen and learn tour with no policy pronouncements. It is a lower key version of Obama's own 2008 trip abroad in which he spoke to a huge throng in Berlin in the heat of that presidential campaign and declared, The walls between old allies on either side of the Atlantic cannot stand.

Romney's visit to the Olympics will allow him to wave the American flag and play up a central piece of his resume - his work to salvage the 2002 Winter Olympics in Salt Lake City.

In London he will meet Prime Minister David Cameron and other officials as well as former Prime Minister Tony Blair to stress the importance of traditional U.S.-British ties. He will also raise some campaign cash from Americans living in Britain.

In Warsaw and Gdansk, Romney will hold talks with Polish officials and former Solidarity leader Lech Walesa and voice support for an ally that has stood as a key bulwark in eastern Europe, a region long dominated by Russia.

The trip is to demonstrate a clear and resolute stand with nations that share our values, said Romney policy adviser Lanhee Chen.

Israel is the most delicate diplomatic stop for Romney, with Syria in turmoil after a bomb attack in Damascus killed the defense minister, and Israeli tensions with Iran rising after a bus bomb in Bulgaria killed six Israeli tourists and prompted Israel to blame Tehran.

'WALKING BETWEEN THE RAINDROPS'

Romney's visit presents him the opportunity to appeal to both Jewish voters and pro-Israel evangelical voters and contrast himself with the Democratic incumbent Obama, who has a rocky relationship with Prime Minister Benjamin Netanyahu.

But it also offers some risks as Romney, a relative novice in foreign policy, may be asked whether he would go to war to halt Iran's nuclear program, a top concern of Israel, and whether he would get the United States involved in the bloody turmoil in neighboring Syria, where President Bashar al-Assad is under siege from rebels.

Romney has sharply criticized Obama's handling of Iran and adamantly declared he would not allow it to possess a nuclear weapon, which Tehran denies seeking. Romney has said that ultimately, regime change is what's going to be necessary.

This has left him open to questions over whether he would launch a military operation against Iran at a time when Americans are weary of wars in Afghanistan and Iraq begun by the last Republican president, George W. Bush.

That's the key consideration, said Martin Indyk, director of foreign policy at the Brookings Institution and author of the book, Bending History: Barack Obama's foreign policy.

On the one hand Romney wants to be able to criticize Obama as being weak on Iran, but I don't think he wants to portray himself as eager for a conflict with Iran. So I think he's walking between the raindrops to try to score Obama without creating the impression that he's keen for another war in the Middle East, Indyk said.

Romney has accused Obama of a lack of leadership for failing to gain U.N. passage of a resolution threatening sanctions against Syria.

Lest Romney face criticism for taking the campaign against Obama overseas, the Romney camp stressed that the candidate will not use the trip to make policy pronouncements.

The friendly imagery from Romney's talks with Netanyahu will be critical in providing a contrast with that of Obama and the Israeli leader, who have clashed in the past.

Obama angered the Israelis a year ago when he embraced a goal long sought by the Palestinians, that the state they seek in the occupied West Bank and Gaza Strip should largely be drawn along lines that existed before the 1967 war in which Israel captured those territories, including East Jerusalem.

The focus of the trip really is about learning, listening ... and it's about continuing to project Governor Romney's strong view that America needs to stand by its allies, particularly allies that are under siege, like Israel, said Romney foreign policy adviser Dan Senor.

Former Secretary of State Condoleezza Rice visited Jerusalem in May and heard complaints from the Israeli leadership about the current state of the U.S.-Israeli relationship, a source familiar with her meetings said.

It has not gone unnoticed there that the Democratic president has not visited Israel, although he has sent some ranking officials there, including Secretary of State Hillary Clinton.

It's not a trivial issue that Obama has not been to Israel during his presidency, said Michael Goldfarb, a Republican foreign policy expert. Of course he went as a candidate, but he ran as a pro-Israeli figure and in office has not lived up to his promises.

Romney, who says Obama has proposed Israel adopt indefensible borders, has known Netanyahu since the 1970s when they both worked briefly at the Boston Consulting Group. Romney also plans to meet Palestinian leaders.

There's no question that Romney needs to be careful, said Shibley Telhami, a Middle East expert at the University of Maryland. He needs to come across as a statesman rather than a panderer and be careful not to lock himself into positions that would tie his hands down the road.

While American Jewish voters overwhelmingly support Democrats, Romney's strong pro-Israeli stance could help him cut into Obama's dominance with this voting bloc, which could make a difference in a battleground state like Florida.

Perhaps more importantly, the Israel trip could energize staunchly pro-Israel evangelical conservatives who have been suspicious of Romney - a Mormon who has a history of being moderate - and had searched for an alternative during the Republican primary battle. (Editing by Deborah Charles and Mohammad Zargham)