This would likely end the chamber's consideration of the nomination for the foreseeable future, Democratic and Republican Senate aides said Monday.
The Senate was scheduled to vote on invoking cloture on the Bolton nomination Monday evening -- and Democratic leadership aides said Minority Leader Harry Reid, D-Nev., controlled enough votes to maintain their weekslong filibuster.
"Mr. Bolton is going to need to work on his own exit strategy," one aide quipped.
A senior Republican aide acknowledged Majority Leader Frist did not appear to have enough votes to push forward with the nomination, and said further action would be unlikely unless Democrats and the White House end their standoff over release of sensitive documents on Bolton to the Senate.
If Monday's cloture vote fails, "the door begins to close on any further Senate floor action, absent a change in the fundamentals," the aide said.
With Senate confirmation unlikely, Bush may opt to appoint Bolton to the job during the July Fourth recess. Secretary of State Condoleeza Rice over the weekend held out the possibility of a recess appointment, and Republican aides have hinted that Bush has considered the possibility.