WASHINGTON — A senior House Democrat who is among his party’s top voices on trade came out against the 12-nation Trans-Pacific Partnership on Thursday, further darkening the congressional outlook for the landmark agreement.

The opposition of the Democrat, Representative Sander M. Levin, who represents suburban Detroit, was not a big surprise, given the objections to the pact by the Ford Motor Company and organized labor. But he had worked with the administration throughout the yearslong negotiations, and his arguments will provide grist for other Democrats to reject the agreement, one of the president’s top priorities.

The Trans-Pacific Partnership, which was signed by the countries’ trade ministers this month after negotiations were completed in October, is not likely to come to a vote in Congress until year’s end, if then. The Senate majority leader, Mitch McConnell, Republican of Kentucky, opposes bringing it to a vote in his chamber before the November elections. People in both parties say the agreement, which would be the largest regional trade alliance in history, currently lacks sufficient support.

Speaking to reporters at a gathering sponsored by The Christian Science Monitor, Mr. Levin said that while his position “has never been ‘no T.P.P.,’ ” he believed that the agreement fell short, particularly in four areas. He cited weaknesses in the protection of worker rights; penalties for nations that manipulate their currencies to underprice their exports; rules that would guard against the use of materials like auto parts from nations — primarily China — that are not parties to the agreement; and a dispute-resolution system that corporations have used to challenge countries’ environmental, health and labor safeguards as anti-trade.