Senate Majority Leader Mitch McConnell (R-Ky.) dealt a significant blow to President Obama’s global trade agenda Thursday, declaring that a sweeping pact with 11 Pacific Rim nations should not be sent to Congress for approval until after the 2016 elections — and maybe not until after Obama leaves office.

McConnell, who previously supported efforts to enhance Obama’s trade negotiating powers, signaled that he was undecided on how he would vote on the deal, but he was clear that the Trans-Pacific Partnership (TPP) would be defeated if it were sent to Capitol Hill next spring or summer, as the administration was planning to do.

“It certainly shouldn’t come before the election. I don’t think so, and I have some serious problems with what I think it is,” McConnell said in an exclusive interview with The Washington Post. “But I think the president would be making a big mistake to try to have that voted on during the election. There’s significant pushback all over the place.”

McConnell’s resistance to moving the pact casts doubt on whether Obama will be able to secure approval for the largest such trade deal ever considered, and one that the president had hoped would be among his final legacy-burnishing efforts. A year into his new role, McConnell also sought to more broadly tamp down expectations for any other breakthrough legislative accomplishments — “I’d be surprised” — before Obama finishes his term in January 2017.

His remarks were the bluntest public statement of doubt on the TPP’s prospects from Republican leadership, which has offered little support since the administration announced in October that a final deal had been reached among the 12 nations.

The U.S. and 11 other nations have come up with a trade deal after years of negotiations. But what's in it, who hates it, and what happens next? (Gillian Brockell/The Washington Post)

The White House had hoped that the GOP’s strong support in June for “fast track” trade legislation, which granted Obama additional powers to complete the pact, would ease the path for final ratification by forging a partnership between the president and his Republican rivals. Obama has championed the trade pact despite fierce opposition from most Democratic lawmakers, labor unions and environmental groups.

“Yeah, I think it would be a big mistake to send it up before the election,” McConnell said in the interview. He noted that the trade authority Congress approved also gives the next president fast-track authority to secure global deals throughout the first term of that administration. “The next president, whoever that is, will have the authority to either revisit this one, if it doesn’t pass, or finish the European deal or other deals, and give Congress a chance to weigh in on it,” McConnell said.

Under the terms of the fast-track legislation, Obama must wait 90 days after the announcement of the final TPP agreement before signing the pact and sending it to Congress; that could happen by Feb. 4. Lawmakers would then work with the administration to determine when a vote would take place. Obama aides have said that could come by late March at the earliest.

“We will continue working with Congressional leaders to pass the Trans-Pacific Partnership as soon as possible next year,” Brandi Hoffine, a White House spokeswoman, said in an email Thursday when asked about McConnell’s comments. “We don’t see any reason to delay the 18,000 tax cuts [through tariff reductions] on Made in America exports in TPP that will benefit our workers and businesses. Our competitors, including China, aren’t standing on the sidelines on trade, we shouldn’t be either.”

But McConnell, who said Thursday that he has relayed his concerns to Obama, is joined in his questioning of the deal by Sen. Orrin G. Hatch (R-Utah), the Senate Finance Committee chairman, who was also a key supporter of the fast-track legislation. They have raised particular concerns about provisions related to tobacco and pharmaceutical companies.

Kentucky is one of the nation’s largest tobacco producers, and Utah has a growing pharmaceutical industry.

Their concerns have dampened enthusiasm among other Republicans, and the debate over trade policies on the 2016 campaign trail has also muddied the prospects for the TPP. Several top contenders for the GOP presidential nomination, including front-runner Donald Trump and Sen. Ted Cruz (Tex.), have denounced the pact, and all of the Democratic candidates, including Hillary Clinton, oppose it.

Even if a more establishment-friendly Republican who supports trade deals emerges as the GOP nominee, the politics of approving the TPP are not likely to significantly improve. The presidential primary race could effectively be over by April, but most states will only then begin to hold their primary contests for congressional seats.

With liberal and conservative activists alike denouncing the deal, some trade supporters in Congress might fear taking that vote just as they face primary challenges.

Obama has continued to try to rally support for the pact, meeting last month with the leaders of the 11 other TPP countries during a visit to Manila.

“We’re going to discuss the road ahead to ensure that TPP is enacted in each of our countries as swiftly as possible,” Obama said before that meeting. “Obviously, execution is critical after we have arrived at the text. . . . This is not easy to do. The politics of any trade agreement are difficult.”

Administration officials said they are concerned that delays in Congress could lead to political complications in other countries with the deal, which was negotiated over more than five years. Some governments could decide to end their participation, especially if U.S. lawmakers begin asking for changes to the accord.

The TPP would lower tariffs on services and goods, including beef, pork, dairy and automobiles, and establish new regulatory provisions for newer industries, including financial services, pharmaceuticals and entertainment production companies. It would also regulate the flow of information and commerce over the Internet.

Obama has said the pact is central to his economic agenda, but it is also viewed inside the administration as an important foreign policy initiative to balance the growing economic clout of China. Although China is not among the TPP nations, Beijing has sought to expand its influence throughout the Asia Pacific.

The president met with several influential business leaders and elected officials at the White House last week. The business community has generally been supportive of the deal, but powerful groups including the U.S. Chamber of Commerce have not formally endorsed the accord.

“We urge quick action,” said Xerox Chairman Ursula Burns, who attended that meeting. “We will put our full weight behind assuring that the members of Congress understand the fact that we really support TPP. Slowing down, in our opinion, doesn’t bring a lot to the table at all. . . . They’re listening, and we have to work hard on it.”

McConnell has balked over a provision that would bar tobacco companies from accessing an international tribunal established to settle disputes between TPP nations and multinational corporations seeking damages for profits lost because of changes in laws — stricter public health regulations on cigarettes, for example. Hatch has been concerned about provisions that would offer pharmaceutical companies that develop next-generation biologic drugs about eight years of protections for intellectual property, four years fewer than is currently available under U.S. laws.

Those two provisions were among the final compromises between the negotiators before the agreement was announced by trade ministers in Atlanta in October.