Representatives from 12 countries pose for a photo after signing the Trans-Pacific Partnership agreement in Auckland, New Zealand. (Michael Bradley/AFP/Getty Images)

Trade ministers from the United States and 11 other Pacific Rim nations formally signed the largest regional trade deal in history on Thursday in New Zealand, but the fate of one of President Obama’s signature economic projects remained fraught on Capitol Hill.

At a ceremony in Auckland, the ministers hailed the Trans-Pacific Partnership (TPP) as a landmark achievement that will bolster trade and investment between nations that make up nearly 40 percent of the world’s gross domestic product. U.S. Trade Representative Michael Froman represented the Obama ­administration.

In a statement, Obama called the TPP “a new type of trade deal that puts American workers first. . . . Put simply, TPP will bolster our leadership abroad and support good jobs at home.”

The signing took place three months after Obama notified Congress of his administration’s intent to sign the accord, satisfying a 90-day public review period required by lawmakers. The next step is for the White House to send implementing legislation to Congress for a vote on final ­ratification.

But after more than six years of negotiations, this final hurdle could be the most difficult for the Obama administration. Republican leaders have cautioned that a vote before the November elections could spell doom for the TPP at a time when many Americans remain uncertain about their economic prospects and leading presidential candidates in both parties have denounced the deal as harmful to U.S. ­workers.

Senate Majority Leader Mitch McConnell (R-Ky.) said a vote should be delayed until the lame duck session of Congress at the end of the year. Such a delay could spell trouble for a pact that required difficult compromises from a dozen nations, each with political uncertainties of its own, and administration officials have said they hope lawmakers would vote by this summer.

“No one should be under any illusions that, because the TPP is being signed today, an up or down vote on the agreement is imminent,” Sen. Orrin G. Hatch (R-Utah), chairman of the Senate Finance Committee, said in a floor speech. “If history has taught us anything, it’s that this process can, and often does, take a very long time to complete. In fact, it’s not an exaggeration — or even all that remarkable — to say that it can take years to get an agreement through Congress after it is signed.”

The administration had hoped that fast-track trade powers approved by Congress after a fierce debate last spring would smooth the path for the TPP. Obama discussed the trade deal with McConnell and House Speaker Paul D. Ryan (R-Wis.) at a meeting at the White House this week, but no timetable for a vote was agreed upon.

“I’m confident at the end of the day because of the strong benefits to the U.S. economy … that members of Congress will see the benefits for their constituents and have the necessary bipartisan support to be approved,” Froman said.

A recent analysis from a pro-trade think tank said the accord would boost U.S. exports and wages, but it also found that 50,000 jobs per year could transition from low-wage, traditional manufacturing to high-tech industries and service sectors. The deal has been endorsed by big business groups, including the U.S. Chamber of Commerce and the National Association of Manufacturers, but most Democrats have opposed it.

Sen. Elizabeth Warren (D-Mass.) said this week that the TPP “is about letting multinational corporations rig the rules — on everything from patent protection to food safety standards . . . to benefit themselves.”

Obama has touted the TPP, which aims to lower trade barriers on goods and services and establish new international commerce regulations, as a key component of his economic agenda and a hedge on China’s growing clout in the Asia Pacific. The other TPP nations are: Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam.

This month, Obama will underscore the accord’s importance when he welcomes the leaders of 10 Southeast Asian nations — including five countries involved in the TPP — at Sunnylands retreat in Southern California.

Ahead of the signing ceremony at the SkyCity Convention Centre in downtown Auckland, Froman and his counterparts met to provide updates on their respective domestic ratification processes. The location for the ceremony was chosen because New Zealand was the nation that first proposed a four-nation regional trade deal in the mid-2000s that ultimately evolved into the TPP.

Outside the meeting, hundreds of protesters gathered to denounce the deal.

In Washington, a group of House Democrats, labor union officials and environmentalists distributed to lawmakers an online petition against the pact purportedly signed by 1 million people in the United States and worldwide.

“This agreement is toxic, and the American people are not buying it,” AFL-CIO President Richard L. Trumka said at a news conference on Capitol Hill. “To all those on the ballot in 2016, we have a simple message: Either you’re with us or you’re against us.”