PRESIDENT OBAMA’S proposed ­Trans-Pacific Partnership (TPP) would connect the United States with 11 other nations in a free-trade area encompassing 40 percent of the world’s economic output. It promises to open Japan’s notoriously closed market to U.S. products more than ever and to cement U.S. leadership in a region that could otherwise fall increasingly under the sway of China. Mr. Obama’s negotiation team is hard at work on the final details, and the White House needs congressional approval of a special law known as trade promotion authority to strengthen its hand in the talks. In his State of the Union address on Tuesday, Mr. Obama urged Congress to pass that measure, which would expedite a vote on the eventual agreement, as soon as possible.

Apparently, Senate Majority Leader Harry Reid (D-Nev.) has other ideas. “I’m against ‘fast track,’ ” he announced Wednesday, using a colloquial term for trade promotion authority. “I think everyone would be well-advised just not to push this right now.” The day after Mr. Obama made his plea, Mr. Reid sounded as if he were rejecting it — thus imperiling the entire TPP project. That might be a stretch: Mr. Reid has never supported trade promotion authority, and he has never been much for free-trade deals, either. He has nevertheless permitted such legislation to move through the Senate in the past, and he stopped short of an explicit threat to block it this time.

Still, Mr. Reid’s remarks emboldened free-trade opponents and gave Republican lawmakers, whose support the president will eventually need, a ready-made excuse not to cooperate. This can’t help but sow confusion among the TPP negotiating partners about the United States’ true intentions and about Mr. Obama’s capacity to work his will. Mr. Reid’s language was ham-handed, given that Mr. Obama is counting on other leaders, especially Japanese Prime Minister Shinzo Abe, to stand up to domestic protectionists. Yet the majority leader’s obvious election-year interest in appeasing opponents of trade promotion authority within his party, notably organized labor, makes it that much harder for Mr. Obama to ask Mr. Abe to take political risks.

In the wake of Mr. Reid’s comment, White House spokesman Jay Carney called the TPP “a very important opportunity to expand trade” and insisted that “the president will continue to press to get it done.” That response was consistent with the administration’s generally low-key lobbying effort with a Congress that hates tough votes on trade. So was Mr. Obama’s brief and mild pitch in the State of the Union. The majority leader’s attitude suggests, however, that the president may need to step up the pressure — and that the Republicans aren’t the only ones on Capitol Hill who can undermine his agenda.