Republicans on Monday fell short of the 60 votes needed in the 100-member Senate to end the bitter debate on Bolton's nomination and pave the way for a confirmation vote.
The White House quickly reacted to the 54-38 vote, deploring the Senate's failure to end the debate.
"John Bolton deserves an up-or-down vote," said White House spokesman Scott McClellan. "He's someone who enjoys majority support, and it's unfortunate that Democrats continue to play politics with the nomination."
Bush could appoint Bolton to the job during Congress' 4 July recess or its month-long August break, enabling the hardline conservative to serve as UN ambassador until January 2007, when a new Congress convenes.
But McClellan declined to say whether Bush would make a recess appointment. Democrats warned such a move could hurt Bolton's standing at the UN.
Earlier, Bush urged the Senate to back a confirmation vote, saying Bolton was the right person for the job at a time the UN is contemplating reform.
"The American people know why I nominated him: Because the UN needs reform," the US president said.
Bush says Bolton is the best man
for the job of envoy to the UN
Republicans, who hold 55 seats in the Senate, had already failed to end the Democrats' stall tactic three weeks ago.
Top Democratic senators say they are willing to allow a vote on Bolton, but only after the US administration provides classified documents that they say are crucial to deciding the nomination.
Bolton has been blocked for weeks by Democrats who opposed everything from past scornful statements about the world body to those who oppose the bullying manner in which he allegedly treated subordinates.
Still others allege he tried to reconfigure intelligence to conform to his political views.
Democrats warned that sending Bolton to the post without a vote of endorsement by the Senate would only serve to further tarnish a nominee whose reputation has already been hurt by the controversy surrounding his past statements and conduct.
The Senate Foreign Relations Committee, charged with vetting Bolton's candidacy, sent his nomination to the full Senate last month without endorsing him after contentious hearings.