A senior Senate Democrat is predicting a “bruising fight” over John Bolton’s second attempt at confirmation as United Nations ambassador when it begins this Thursday, but the party leadership has not committed to mounting another filibuster.

Senator Dodd of Connecticut yesterday became the second Democratic member of the Senate Foreign Relations Committee to vow renewed opposition to Mr. Bolton, who has been serving as the U.N. envoy since Mr. Bush appointed him during a Senate recess nearly a year ago.

“No, this is going to be a bruising fight,” Mr. Dodd said on CNN’s “Late Edition,” when asked whether he would vote to confirm Mr. Bolton. “I regret this. I’m sorry the administration wants to go forward with this.”

The Senate Foreign Relations Committee is pressing ahead with Mr. Bolton’s renomination after a key Republican member, Senator Voinovich of Ohio, said last week that he had changed his mind and would support confirmation. A hearing is set for Thursday, although a full floor vote is not expected before September. Mr. Voinovich was instrumental in helping Democrats block Mr. Bolton’s confirmation last year.

Mr. Bolton’s recess appointment expires with the adjournment of Congress at the end of the year, but the president could reappoint him between sessions. Democrats held up his initial nomination over concerns about his temperament and past criticism of the United Nations, along with the White House’s refusal to release certain documents from earlier in his career.

While Mr. Dodd said the “problems still persist” with Mr. Bolton, observers say the political landscape may have changed and that centrist Democrats may be reluctant to pick another fight over the U.N. post with midterm elections looming and a war escalating in the Middle East.

In publicly opposing Mr. Bolton, Mr. Dodd joined Senator Biden of Delaware, the ranking Democrat on the Foreign Relations Committee, who last week said Mr. Bolton’s performance at the United Nations “confirms my conviction that he is the wrong person for the job.”

The Senate minority leader, Harry Reid of Nevada, has not indicated what his party will do.

A spokesman for the senator, James Manley, said yesterday that Mr. Reid would consult with members of the Foreign Relations Committee and other Democrats before deciding how to proceed. Still, Mr. Reid “has serious concerns about John Bolton’s qualifications for the job,” Mr. Manley said.

The decision over Mr. Bolton’s confirmation may be particularly difficult for lawmakers who are supportive of Israel, including Senators Schumer and Clinton. Neither New York senator has said whether they would back a filibuster, and repeated calls and e-mails to their spokesmen were not returned yesterday.

Mr. Bolton, in an appearance on Fox News yesterday, said Mr. Voinovich’s support “represents a fairly dramatic change in the political dynamic in the Senate.” Saying he hoped to get a floor vote, Mr. Bolton cited the backing of all Senate Republicans and “a number of Democrats,” although he did not name which ones.