Fifty-nine former US diplomats have written to the chairman of a key Senate committee in protest at the nomination of John Bolton as ambassador to the UN.
The State Department describes Mr Bolton as a "great nominee".
The diplomats, who served under both Republican and Democrat presidents, described Mr Bolton, a known critic of the UN, as "the wrong man" for the job.
They urged the Senate Foreign Relations Committee to block his appointment.
Mr Bolton served as under-secretary of state responsible for arms control during President Bush's first term.
Chief among the objections was Mr Bolton's stated view that the UN "is valuable only when it directly serves the United States".
In addition, Mr Bolton was criticised for his record as US arms control supremo.
He had an "exceptional record" of undermining potential improvements to US national security through arms control, the diplomats complained.
Among the most senior signatories was Arthur Hartman, former ambassador to France and the Soviet Union under Presidents Jimmy Carter and Ronald Reagan and assistant secretary of state for European affairs under President Richard Nixon.
Princeton Lyman, a former ambassador to South Africa and Nigeria, Monteagle Stearns, US representative in Greece and Ivory Coast, and Spurgeon Keeny Jr, Jimmy Carter's deputy director of arms control, also signed the letter.
Mr Bolton requires approval from the foreign relations committee - made up of 10 Republicans and eight Democrats - before being told he can head to the UN's New York headquarters.
Announcing his nomination at the start of March, US Secretary of State Condoleezza Rice described him as a "tough-minded diplomat" with "a proven track record of effective multilateralism".
But the former diplomats insist his hard-line views on states such as Cuba and Syria, as well as previous paid employment for the government of Taiwan, make him an unsuitable candidate.
"Given these past actions and statements, John R Bolton cannot be an effective promoter of the US national interest at the UN," they wrote.
"We urge you to oppose his nomination."