A Senate committee has sent John Bolton's nomination to be US ambassador to the United Nations to the full Senate.
But he does not carry an endorsement after a key Republican branded President George W Bush's choice as unfit for the job.
Although the committee action advanced Mr Bolton's nomination, Republican Senator George Voinovich's criticisms of Mr Bush's choice for UN envoy gave Democrats more ammunition to fight his confirmation in the full Senate on the grounds he was a bully who tried to mould US intelligence to fit his views.
Republicans said Mr Bolton's prospects were good in the Senate, which they control 55-45, but Mr Voinovich's criticism and the rare lack of a committee endorsement were embarrassments for Mr Bush who has worked aggressively for Mr Bolton, a favourite of conservatives.
The committee voted 10-8, along party lines, to advance the nomination. Democrats argued that the committee should reject Mr Bolton (56) and force Mr Bush to find a better candidate.
The White House said it was confident the Senate would back Mr Bolton, who Mr Bush has touted as the best choice to push for reforms at the world body.
Mr Voinovic had condemned Mr Bolton as "the poster child of what someone in the diplomatic corps should not be.
"The United States can do better than John Bolton," the Ohio senator said. He said the blunt Mr Bolton could worsen the United States's already poor image around the world.
It was unclear when the Senate would take up the nomination. Democrats also said they were considering filibustering it, which would force Republicans to muster 60 supporting votes, but most said that was unlikely.