President Bush yesterday nominated John C. Danforth, the former Republican senator from Missouri who has most recently served as special envoy to Sudan, to be the U.S. ambassador to the United Nations. If confirmed, he will replace John D. Negroponte who becomes top envoy to Iraq after the U.S.-led occupation ends June 30.

An ordained Episcopal minister and an heir to the Ralston-Purina Co. fortune, Danforth is a widely respected politician often referred to as "Saint Jack" during his Senate years who appeals to the religious right as well as some Democrats. But he has limited diplomatic experience, warn former colleagues and friends. Filling the shoes of Negroponte, a deft career diplomat who had major impact at the United Nations, will not be easy for the Missouri politician, they say.

"He hasn't had any great experience in diplomacy, but knowing how to work the crowd in the U.S. Senate teaches you how to work the crowd anywhere," said former ambassador Robert Oakley, who worked with Danforth's peace mission in Sudan and has talked with him about the U.N. nomination. "He doesn't know much about the U.N., but he's a quick study and has a good staff. He is looking forward to the challenge. . . . I suspect he will be looking to show that [the] U.S. can work with others."

Democrats also voiced support for the surprise nominee. "He is a terrific choice, a moderate and conciliatory man. His senatorial skills will work well at the United Nations," Richard C. Holbrooke, U.N. ambassador during the Clinton administration, told Reuters.

A former senior administration who worked with Danforth said he is a "talented guy who's been around" but also expressed concern that he is not imaginative in either diplomacy or peace-making, and may not pick up the nuances that are pivotal in diplomatic dealings.

For all Danforth's good intentions, he is likely to find representing the United States at the world body the most difficult assignment of his career, added the former official, who requested anonymity as the nomination is pending.

He will present U.S. positions on contentious issues to a world body that includes many nations who are skeptical of, if not hostile to, the Bush administration foreign policy.

Danforth, now a lawyer in St. Louis, was also criticized for a Senate vote against imposing sanctions on South Africa's apartheid regime in the mid-1980s and a vote cutting funds for U.N. peacekeeping in the 1990s. He also voted to limit U.S. support for international family planning, said Don Kraus, vice president for Citizens for Global Solutions, a nonpartisan group that works to build U.S. support for international institutions.

Besides 18 years in the Senate from 1976 to 1995, Danforth is best known for using a reputation for personal integrity to resolve domestic and international conflicts.

In 2001, Bush nominated Danforth to help mediate the decades-old Sudan civil war. Oakley described him as a "man of principle who got something done that no one else could do and did it quietly."

But foreign policy experts said Danforth often did not personally engage. "His long experience in brokering agreements in the Senate should enable him to be confirmed faster than many others, but some of his limitations came into view in Sudan, including lack of engagement in details of the negotiations which he left to staff people and could be a liability at the U.N.," said John Prendergast, special adviser on Africa to the International Crisis Group, a nonpartisan conflict monitoring group.

Danforth almost personally salvaged the Supreme Court nomination of Clarence Thomas by defending his protege after Thomas was charged with sexual harassment by former employee Anita F. Hill. Thomas, a conservative black Republican lambasted by civil rights groups, had worked as an assistant to Danforth when he was Missouri's attorney general. Both were Yale Law School graduates.

In his book "Resurrection: The Confirmation of Clarence Thomas," Danforth described praying with Thomas and their wives in his office and playing "Onward Christian Soldiers" on a tape recorder before Thomas made his final defense at the Senate Judiciary Committee.

Danforth, who was briefly considered as a running mate by the Bush campaign in 2000, was appointed by the Clinton administration to head an inquiry into the 1993 deaths of 80 Branch Davidians in Waco, Tex.

After a 14-month investigation costing $17 million, Danforth concluded the FBI had "nothing to hide." U.S. agents had not fired into the Davidian complex and were not responsible for the subsequent fire. But he did criticize FBI and Justice Department personnel for lack of cooperation with his investigation, which he said undermined public confidence in government.

John C. Danforth may find the U.N. post the most difficult of his career, a former official said.