"My concern is that at the moment of the greatest need for diplomacy in our recent history, we are not particularly effective at it," said Sen. Joseph Biden of Delaware, the Foreign Relations Committee's top Democrat.
Biden also has said Bolton should not get a confirmation vote until the White House turns over documents he requested when Bolton was nominated last year.
Another committee Democrat, Sen. Chris Dodd of Connecticut, made clear Bolton's work at the U.N. hasn't changed his mind, CBS News correspondent Bob Fuss reports.
Dodd cited recent press reports on Bolton's interactions with other U.N. officials as evidence that Bolton "clearly has an aversion, in my view, to building consensus."
A Senate Foreign Relations Committee confirmation hearing for Bolton, who has been serving as ambassador under a recess appointment, twice was interrupted by protesters opposing his confirmation.
Shrugging off the criticism of Bolton, Republicans predicted that the U.N. ambassador would be confirmed soon for the position on a permanent basis. The White House on Thursday praised the work Bolton has done so far.
"We think Ambassador Bolton has done a terrific job," White House press secretary Tony Snow said. "He's won over a lot of critics while building alliances on a range of issues, including Iran and North Korea, and working tirelessly to achieve meaningful results on reforms at the United Nations."
Mr. Bush last year temporarily installed Bolton as U.N. ambassador while Congress was in recess, an appointment that will expire in January. The recess appointment, provided for by the Constitution, came after Democrats blocked repeated attempts by GOP leaders to grant Bolton Senate approval.
A Republican on the panel who helped sink Bolton's confirmation last time, Ohio Sen. George Voinovich has changed his mind, saying he'll support Bolton now, making his confirmation more likely.
"The sole thing that remains is the constitutional authority of the Senate to give its advice and consent," said Sen. John W. Warner, R-Va.
"I do believe without any reservation whatsoever that the Senate will and should give that advice and consent to this nominee because he becomes an integral member of the president's national security team at a time when our nation is faced with these many complex issues," Warner added.
Speaking from prepared testimony, Bolton called for a "durable solution" to the violence in the Middle East and the need to "defang" Hezbollah.
"We are actively considering a variety of methods" to disarm Hezbollah, including establishing an international security force in the region, he said.
Bolton also said the United States remains committed to bringing peace to Darfur.
Bolton said some "modest progress" has been made in U.N. reform. "The goal now is to identify priority targets where progress can be made" and create a "lasting revolution of reform," he said.
Sen. Richard Lugar, chairman of the Senate Foreign Relations Committee, said Bolton exhibited "experience and accomplishment" and noted the Senate has already conducted an "exhaustive review" of Bolton's credentials.
By resubmitting Bolton's nomination to the Senate, the president has made clear "that Ambassador Bolton is important to the implementation of U.S. policies at the United Nations and to broader U.S. goals on the global stage," said Lugar, R-Ind.