WASHINGTON - Passing over better-known candidates, President Barack Obama on Friday nominated global health expert and Dartmouth College President Jim Yong Kim to lead the World Bank. It was a surprise pick aimed in part at fending off challenges from developing nations eager to end the U.S. monopoly of the top job at the international institution.

Obama's appointment all but guarantees that Kim, a 52-year-old physician and pioneer in treating HIV/AIDS and tuberculosis in the developing world, will take over at the helm of the World Bank. Though he was born in South Korea, he will extend a tradition of American presidents dating back to the organization's founding in 1944.

The 187-nation World Bank focuses on fighting poverty and promoting development. It is a leading source of development loans for countries seeking financing to build dams, roads and other infrastructure projects.

Several developing nations had sought to break the U.S. leadership streak when current Bank President Robert Zoellick announced he would step down at the end of June. That put Obama in the delicate position of balancing his desire to see emerging economies step forward on the world stage and the pressures of an election year. His support for a non-U.S. candidate could have provoked fresh criticism from Republicans, who frequently question whether Obama believes in the notion of "American exceptionalism."

As Obama announced Kim's nomination from the White House Rose Garden on Friday morning, he tried to make the case that an American with a unique background and broad international experience would be a committed representative of the developing world's interests.

"Jim has truly global experience. He has worked from Asia to Africa to the Americas, from capitals to small villages," Obama said. "His personal story exemplifies the great diversity of our country."

In addition to Kim, Obama was joined in the Rose Garden by Treasury Secretary Timothy Geithner and Secretary of State Hillary Rodham Clinton, both of whom could have had the job but were not interested. Senior administration officials said it was Clinton who first recommended Kim to Obama.

Obama, aware of the concerns of the developing world, expanded his search beyond the usual slate of high-ranking government officials and prominent business leaders. Officials described internal White House strategy only on condition of anonymity.

"It's time for a development professional to lead the world's largest development agency," Obama said as he announced the nomination.

The World Bank's 25-member executive board will officially select a new president next month. But given that the U.S, as the world's largest economy, has the largest percentage of the votes, Kim is expected to prevail.

He was widely praised by officials in the U.S. and overseas. Former President Bill Clinton, who advocated for Kim during Obama's selection process, said in a statement that the nominee was "an inspired and outstanding choice." Rwandan President Paul Kagame said Kim was "a true friend of Africa" and "a leader who knows what it takes to address poverty."

Still, Kim's nomination was expected to face some resistance from those who believe it's time for the developing world to take the reins at an organization focused on addressing its needs.

Officials at Oxfam, the international aid agency, urged the World Bank to welcome a genuine debate about its leadership and not just rubber-stamp the U.S. selection.

"It's time for a symbolic break from the past and for a new leader to steer the Bank into the 21st century," Oxfam spokeswoman Elizabeth Stuart said.

The argument from the developing world was one Obama himself often has made in his travels as president to places like China, India and Brazil. Rising powers, he has said, must play a larger role on the world stage.

Developing countries put forward two candidates, including Ngozi Okonjo-Iweala, Nigeria's finance minister, who has the backing of the three African countries on the World Bank board. Jose Antonio Ocampo, a Columbia University professor who had been finance minister for Colombia, was also nominated.

Jeffrey Sachs, a prominent development economist at Columbia University in New York, personally campaigned for the World Bank post and had been expected to be nominated by Italy's representative on the World Bank board. But Sachs announced on his Twitter account Friday that he was withdrawing in favor of Kim.

"Jim Kim is a superb nominee for the WB. I support him 100 percent. I thank all who supported me and know they'll be very pleased with today's news," Sachs said in a message.

Kim is expected to travel around the world on a listening tour to rally support for his nomination ahead of the World Bank vote, Obama administration officials said.

Obama picked Kim over several more well-known candidates, including Susan Rice, current U.S. ambassador to the United Nations; Sen. John Kerry, D-Mass., and Lawrence Summers, former director of the president's National Economic Council.

Others mentioned for the World Bank post included Indra Nooyi, the head of PepsiCo, and Laura D'Andrea Tyson, who served in top economic jobs in the Clinton administration.