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, New Zealand, the ministers hailed the Trans-Pacific Partnership as a landmark achievement that will bolster trade and investment among nations that make up nearly 40 percent of the world’s gross domestic product.

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.