WASHINGTON, April 21 - President Bush issued a strong new defense today of John R. Bolton, his nominee as ambassador to the United Nations, even as associates of Colin L. Powell, the former secretary of state, said that Mr. Powell had expressed reservations about Mr. Bolton in conversations with at least two wavering Republican senators.

The associates said that in private telephone conversations Mr. Powell had made clear his concerns with Mr. Bolton on several fronts, including his harsh treatment of subordinates. The associates said that Mr. Powell had also praised Mr. Bolton's performance on some matters during his tenure as undersecretary of state, but they also said that Mr. Powell had stopped well short of the endorsements offered by President Bush and by Mr. Powell's own successor as secretary of state, Condoleezza Rice.

The accounts of Mr. Powell's private message about Mr. Bolton suggested a new gulf between the former secretary of state and the president, who spoke out forcefully today in defense of Mr. Bolton. In a speech here, Mr. Bush portrayed Democratic opposition to Mr. Bolton as being politically driven, and urged the Senate "to put politics aside and confirm John Bolton to the United Nations."

Mr. Bush's comment and others by a White House spokesman suggested that the administration was determined to defend Mr. Bolton's nomination, despite crumbling support among Senate Republicans that has left the nomination in peril.

Indeed, Mr. Bush showed no sign of backing away from the Bolton nomination. Speaking before the Independent Insurance Agents and Brokers of America, he brought up the subject in the first moments of his address, saying, "I welcome you to the nation's capital, where sometimes politics gets in the way of doing the people's business."

"Take John Bolton, the good man I nominated to represent our country at the United Nations," Mr. Bush said.

"John's distinguished career in service to our nation demonstrates that he is the right man at the right time for this important assignment. I urge the Senate to put politics aside and confirm John Bolton to the United Nations."

Mr. Powell has not spoken publicly about the Bolton nomination. But his associates said he had told Senators Lincoln Chafee of Rhode Island and Chuck Hagel of Nebraska, in response to questions, that he had been troubled by the way that Mr. Bolton had treated an intelligence analyst and others at the State Department who disagreed with him.

Mr. Chafee and Mr. Hagel are both Republican members of the Senate Foreign Relations Committee, and both have expressed concern about Mr. Bolton's temperament, credibility and treatment of intelligence analysts. The senators' concerns, along with those of Senator George Voinovich, an Ohio Republican, were among the factors that have forced the Senate Foreign Relations Committee to postpone until next month a vote on Mr. Bolton's nomination.

There were conflicting accounts as to whether Mr. Powell or the senators had initiated the phone calls. A spokeswoman for Mr. Powell said he had only returned calls from others, but one person familiar with one conversation said it had been Mr. Powell who had reached out to Mr. Hagel.

In testifying against Mr. Bolton's nomination, Carl W. Ford Jr., a former assistant secretary of state, told the committee that Mr. Powell had acted in 2002 to reassure intelligence analysts troubled by Mr. Bolton's harsh treatment of one of their colleagues, Christian Westermann, in a dispute related to Cuba. Mr. Powell's former chief of staff, Lawrence Wilkerson, said in an interview this week that Mr. Bolton would be an "abysmal ambassador" to the United Nations.

Earlier this month, five former Republican secretaries of state signed a letter to the Senate committee that had endorsed Mr. Bolton's nomination, but Mr. Powell was not among them. In a telephone conversation with Mr. Chafee, the associates said, Mr. Powell said he had not joined in the endorsement in part because he did not normally sign group letters, but also because he believed such endorsements were appropriate only in cases where his point of view was clear-cut.

Told of the accounts provided by Mr. Powell's associates, a spokeswoman for Mr. Powell, Peggy Cifrino, said in an e-mail message: "To be precise, General Powell has returned calls from senators who wanted to discuss specific questions that have been raised. He has not reached out to senators. The general considers the discussions private."

Mr. Powell served as secretary of state under Mr. Bush for nearly four years, and had told associates in 2004 that he was looking forward to returning to private life. But Mr. Powell was described by some associates as hurt that Mr. Bush, in selecting Ms. Rice as the new secretary, had not asked Mr. Powell if he wanted to stay on. Mr. Powell remains highly regarded by many moderate Republicans, but as secretary of state, his relationship with Vice President Dick Cheney was notably strained, according to many accounts, including a detailed narrative in the book "Plan of Attack," by Bob Woodward of The Washington Post.

Mr. Cheney is now regarded as Mr. Bolton's chief patron within the administration, and some administration officials say he has strongly resisted the idea that the White House may withdraw the Bolton nomination in the face of Democratic complaints and Republican wavering. The chief White House spokesman, Scott McClellan, said today that the White House would try vigorously to answer any questions that Republican senators had about Mr. Bolton's nomination, and he dismissed as unsubstantiated the accusations that Mr. Bolton had behaved inappropriately in his dealings with intelligence analysts and other subordinates.

In a brief interview this week, Mr. Chafee declined to discuss any conversation with Mr. Powell, saying that "I'm going to keep some things confidential." A spokesman for Mr. Hagel, Mike Buttry, would say only that "Senator Hagel and Secretary Powell speak frequently about a lot of things," adding, "Senator Hagel doesn't comment on their private conversations."

A spokesman for Senator Richard Lugar, Republican of Indiana, chairman of the Foreign Relations Committee, said that Mr. Lugar had not spoken with Mr. Powell to discuss the Bolton nomination.

The associates of Mr. Powell who discussed the matter did so in response to repeated questions in recent days. They would not allow their names to be used, saying that they did not want to add to tensions between Mr. Powell and the White House, but said they were willing to discuss the matter in order to provide an accurate account of Mr. Powell's views.

One associate said that Mr. Powell had used at least one of the conversations to say that Mr. Bolton had worked "fairly well" with Mr. Powell on several issues, including those related to Iran, an effort known as the Proliferation Security Initiative, and the phasing out of the Anti-Ballistic Treaty with Russia and the phasing in of an alternative known as the Moscow Treaty. But associates said Mr. Powell had also made clear that Mr. Bolton had "had problems" with Mr. Westermann and others who disagreed with him.

"In short, he gave the senator a balanced appraisal of Bolton," Mr. Powell's associate said of one call, between Mr. Powell and Mr. Chafee.

Staff members of the Foreign Relations Committee were working today to strike an agreement between Democrats and Republicans on a plan to seek further information about a number of disputed issues related to Mr. Bolton, including his requests to the National Security Agency for identifying information about American officials mentioned in communications intercepted by the agency.