Mitt Romney’s overseas trip was designed to reassure voters — proving that a businessman candidate with no diplomatic experience could still play the role of statesman.

It was not a complete success.

Instead, Romney managed to offend at two of his three stops, angering British leaders in London and Palestinian leaders during a visit to Israel. For some observers, Romney’s unforced errors raised questions--instead of answering them--about the readiness of the candidate and his campaign team.

Now, Romney will return to the U.S. with a different job than he wanted. Instead of enjoying an afterglow from the trip, his next task will be to change the subject, possibly by announcing his pick for vice president.

“He kept tripping over the message,” said Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies. Alterman added: “It felt like—for somebody who’s been campaigning for year—he’s still got a learning curve.”

On Tuesday, Romney had what might have been his most successful day of the trip, giving a stirring speech in Warsaw on the historical bonds and democratic ideals that bind America and Poland.

But even on that day, his famously well-oiled campaign had its own ugly moment. A spokesman told reporters to “Kiss my ass” and “Shove it” when they shouted questions at the candidate. The aide later apologized.

And Romney himself was still stuck trying to limit the damage from his remarks a day earlier.

Romney had seemed to suggest during a speech at a Jerusalem fundraiser that the economy of the Palestinian territories lagged behind Israel’s in part because of “culture.” The former Massachusetts governor said in a Fox News Channel interview Tuesday that he “did not speak about the Palestinian culture or the decisions made in their economy.”

“That’s an interesting topic that perhaps could deserve scholarly analysis, but I actually didn’t address that. Certainly don’t intend to address that during my campaign,” he said, according to a partial transcript released by Fox News Channel. “Instead, I will point out are that the choices a society makes have a profound impact on the economy and the vitality of that society.”

He called the media focus on his remarks an attempt by some to “divert from the fact that these last four years have been tough years for our country.”

Still, even one of Romney’s allies in the Senate said that the Jerusalem incident showed that Romney was still adjusting to the role of Republican front-runner.

Sen. Lindsey Graham (R-S.C.) said that Romney’s comments about “culture” in Israel missed important nuances about that region. He said that one Palestinian area, the West Bank, was growing quickly, while the Gaza Strip--controlled by the militant group Hamas--was stagnating.

Romney should have made that distinction clear, he said, instead of describing all Palestinians together.

“Words are important. I think you, as a candidate for president, you’ll soon understand that what you say is different,” Graham said. “That’s a good experience for him.”

Romney’s comments came during a talk to campaign donors in Jerusalem. The candidate said he had sought to understand why Israel’s gross domestic product was so much higher, on a per-capita basis, than the same figure in Palestinian areas.

Romney said he had studied a book called “The Wealth and Poverty of Nations,” searching for an answer about why two neighboring places like this — he also cited the U.S. and Mexico and Chile and Ecuador — could have such disparate prosperity.

“Culture makes all the difference. Culture makes all the difference,” Romney said, repeating the conclusion he drew from that book, by David Landes. “And as I come here and I look out over this city and consider the accomplishments of the people of this nation, I recognize the power of at least culture and a few other things.”

Romney’s remark, and the resulting hubbub, showed the difficulties of saying anything new about the long-running struggle between Palestinian Arabs and Israelis.

Elliot Abrams, a conservative senior fellow at the Council on Foreign Relations, said that Romney might have elaborated on his point about Palestinian “culture,” by noting that the economy may be hurt by a comparative lack of women in workplaces and schools.

“It wouldn’t have made his remarks any more right or wrong,” Abrams said. He believes Romney has been unfairly criticized for the remark in the media: “It would have just been playing defense against attacks from Arab spokesmen, or American reporters.”

On Tuesday, Romney’s chief strategist told reporters that the candidate’s supposed gaffes were simply evidence that Romney is a straight shooter, unafraid of the truth

“He has a tendency to speak his mind and to say what he believes and whenever you do that there will be those that disagree with you and there will be those that agree with you,” strategist Stuart Stevens told reporters. “That’s what he’s done in these situations. I think people like that. I think that this idea that you have to not speak your mind is something that’s not very appealing to people.”

But President Obama’s campaign clearly believed it could use Romney’s own overseas trip against him. In a conference call with reporters, campaign senior advisor Robert Gibbs said that Romney had failed his own test.

“Governor Romney showed on this trip that he may not have the discipline to handle these delicate diplomatic interactions,” said Gibbs, who previously served as Obama’s White House Secretary. “He was simply unable to represent America on the world stage.”

Meanwhile on Tuesday, Democrats announced that San Antonio Mayor Julian Castro will be the keynote speaker at their September convention. The choice of the 37 year-old Castro is the latest signal of President Obama’s efforts to win over Latino voters in his close race against Romney.

Castro was first elected mayor of San Antonio in 2009. His mother was a prominent Mexican-American activist in San Antonio, and his twin brother Joaquin is a Democratic state representative who is now running for Congress. Julian Castro, a graduate of Harvard Law School, is known for a more pragmatic style of governing, eschewing his mother’s more fiery, left-wing political activism.

Also, Democrats said that first lady Michelle Obama will speak on the first night of their national convention, Sept. 4.

Speaking at the University of Warsaw Tuesday, Romney recounted Poland’s fights against Nazis in World War II, and against Soviet-backed communist leaders during the Cold War. It was a stirring speech about the acts of bravery and fortitude that mark Polish history.

But Romney, of course, is not running for president of Poland. Instead, his speech seemed to be meant as a strong — if indirect — criticism of President Obama’s economic policies back home.

Romney said Poland’s modern leaders were battling the same kind of ideas he has previously blasted Obama for embracing, including a willingness to increase debts and spending. Romney said a Polish leader told him during his visit that his country’s economic philosophy is, “You don’t borrow what you can’t pay back.”

“The false gods of the all-powerful state claim the allegiance of a lonely few,” Romney said, according to prepared remarks distributed by his campaign. “It is for us, in this generation and beyond, to show all the world what free people and free economies can achieve for the good of all.”

On Tuesday morning, as Romney walked back to his vehicle following a visit to the Tomb of the Unknown Soldier in Warsaw’s Pilsudski Square, Romney ignored reporters’ questions and aides scolded reporters for showing a lack of respect for a Polish historical site.

When one reporter noted that Romney has not taken questions from his traveling press corps except for three questions at a brief media availability last Thursday in London, Romney spokesman Rick Gorka said, “Kiss my a--. This is a holy site for the Polish people. Show some respect.”

Moments later, Gorka told another reporter to “shove it.”

Gorka later called the reporters to apologize.

Also on Tuesday, Romney’s campaign released a “Mitt’s VP” mobile app, which would allow users to learn immediately when Romney selects a running mate. Romney is expected to make that pick sometime after his return from the European trip, and before the Republican convention begins in Tampa on Aug. 27.

Also Tuesday, Romney’s campaign released a minute-long ad, in which the candidate speaks directly to the camera. This is part of an effort by Romney to introduce himself to voters.

“I know what it’s like to hire people and to wonder whether you’re going to be able to make ends meet down the road,” the candidate says from the front wheel of an SUV. “From those experiences, I went off to have the chance at running the Olympics in Salt Lake City in 2002.”

Rucker reported from Warsaw. Staff writers Felicia Sonmez and Rachel Weiner also contributed to this report.