President Obama's high-stakes bid to complete one of the largest free trade pacts in U.S. history--over the objections of most Democrats--moved ahead Thursday when the Senate introduced bipartisan legislation that would give his administration vast new powers to close the deal.

The fast-track trade bill from Sens. Orrin G. Hatch (R-Utah) and Ron Wyden (D-Ore.) would allow the president to present a final agreement to Congress for an up-or-down vote without lawmakers being able to amend the terms. A similar bill, drafted by Rep. Paul Ryan (R-Wis.), is expected to be introduced in the House in the coming weeks.

Hatch, chairman of the Senate Finance Committee, said he would hold a markup and vote on the bill April 23.

Obama administration officials have called fast-track authority crucial to wrapping up negotiations on the Trans-Pacific Partnership (TPP), a 12-nation trade and regulatory deal in the Asia Pacific that Obama has touted as a cornerstone of his second-term agenda.

"My top priority in any trade negotiation is expanding opportunity for hardworking Americans," Obama said in a statement Thursday.  "It’s no secret that past trade deals haven’t always lived up to their promise, and that’s why I will only sign my name to an agreement that helps ordinary Americans get ahead. At the same time, at a moment when 95 percent of our potential customers live outside our borders, we must make sure that we, and not countries like China, are writing the rules for the global economy."

The president has said the deal would help ensure U.S. competitiveness in the face of a rising China. But getting it across the finish line on Capitol Hill will mark a major leadership test for Obama. The deal faces fierce opposition from many Democrats, labor unions and environmental groups. They said the TPP will kill U.S. jobs and benefit large, multinational corporations.

The AFL-CIO announced Thursday it would launch a six-figure ad buy to fight the fast-track bill in a series of digital ads targeting 16 senators and 36 House members. The campaign could expand to newspapers and television, the organization said.

The fast-track bill, formally known as "trade promotion authority," was negotiated for months between Hatch and Wyden, with the Democrat under increasing pressure from liberal groups not to sign onto the legislation.

Presidents have over the past 40 years enjoyed fast-track powers on trade, but the powers have not been renewed since they expired in 2007 under George W. Bush. Obama and his aides have said they need the authority because other countries are unwilling to agree to final terms if they believe Congress will amend the pact afterward.

"We must speak with one voice in our demands and provide assurance that we will deliver what we promise," Hatch said.

Wyden fought for, and won, a number of provisions aimed at ensuring greater public transparency in the trade negotiations, as well as a key provision that would allow the Senate to turn off the fast-track authority if the Obama administration fails to live up to certain requirements during the negotiations.

"One issue that has come up again and again is the excessive secrecy that seems to have accompanied so much of this debate," Wyden said Thursday. "American trade policy needs to be debated openly."

The bill also includes provisions mandating that the administration pursue protections for workers and the environment and a stipulation that U.S. negotiators make human rights a priority, though that provision lacks specifics.

Politically, the legislation places Obama squarely against a majority of House Democrats and a large bloc of Senate Democrats, many of whom believe previous trade deals hurt U.S. workers. In a sign of how difficult the fight will be, Sen. Charles E. Schumer (D-N.Y.), who is expected to take over as leader of his party in the Senate in 2017, said he was skeptical of the TPP because it lacks protections against currency manipulation.

"All evidence I’ve seen is that this hurts middle-class incomes,"  he said at a hearing before the trade bill was introduced. "I can’t be for it."

Later Thursday, Schumer said that support from him and many Democrats would depend on a separate currency manipulation bill that Hatch has promised to bring up, saying he would not settle for a "milquetoast" measure.

Hatch made clear that he'll give Schumer the chance in committee, but that TPP would move on its own track. "We'll do a bill on currency manipulation, but it can't be part of this, or it's dead. So we can't kill trade just because somebody wants one aspect or another," Hatch said.

Sen. Richard J. Durbin (D-Ill.), the minority whip, estimated that only a quarter of Senate Democrats were in the president's camp right now. He broke down his caucus ranks this way: "A fourth, hell no; a fourth, lean yes; and a big group undecided."

With most Senate Republicans likely to support the fast-track legislation, most corporate and union officials expect the Senate to clear the bill in May and set up a much bigger fight in the House, where the margins are incredibly narrow at the moment.

Several dozen House Republicans either come from manufacturing districts and do not support trade deals, or are just so opposed to giving Obama any new authority that they would vote no. That means Obama would need to deliver more than 30 Democrats to win passage in the House, which insiders on both sides of the aisle say is not guaranteed.

"This is wrong, and members of Congress will not stand for it," said Rep. Rosa L. DeLauro (D-Conn.), who has led the liberal opposition to trade deals.