Senior U.S. lawmakers reached agreement on Thursday on a bill to give the White House "fast track" authority to negotiate a trade pact with 11 other Pacific nations that is central to President Barack Obama's strategic shift toward Asia.

The agreement sets the stage for a tough legislative battle over Obama's proposed Trans-Pacific Partnership (TPP), which would connect a dozen economies by cutting trade barriers and harmonizing standards in a deal covering two-fifths of the world economy and a third of global trade.

The bill gives lawmakers the right to set negotiating objectives but would restrict them to a yes-or-no vote on trade deals such as the ambitious TPP, a potential legacy-defining achievement for the Obama administration.

The White House has faced pressure to make progress on the bill ahead of a meeting between Obama and Japanese Prime Minister Shinzo Abe on April 28 in Washington.

Japanese and U.S. officials met this week in Tokyo in a bid to strike a two-way deal giving momentum to the pan-Pacific pact. Japanese officials have said success depends on whether the U.S. Congress approves measures to ease passage of trade deals, or trade promotion authority (TPA).

"This is a smart, bipartisan compromise that will help move America forward," Republican Senate Finance Committee Chairman Orrin Hatch said after leaders of Congress's tax-writing committees reached agreement on the legislation, which will be introduced in the Senate and House of Representatives.

Trade associations and lobbyists welcomed the bill's introduction but said it should be passed quickly and that it could falter in the face of Congressional and partisan politics. Unions immediately condemned it and announced a new advertising campaign to pressure lawmakers.

TPP must pass Congress this year to avoid being bogged down in the run-up to the 2016 U.S. elections.

FAST-TRACK

Japan and other TPP partners have said it is vital to have fast-track authority, which gives trading partners certainty that agreements will not be picked apart.

The deal between Hatch and the panel's top Democrat, Ron Wyden, to move Trade Promotion Authority ahead in tandem with a bill to extend support for workers hurt by trade is no guarantee legislation will pass Congress. Opponents are lobbying hard to defeat it and many Democrats are still undecided.

"You bring up TPA in the House today, the best you would have is a handful of Democrats," Sander Levin, the top Democrat on the House of Representatives committee responsible for trade, said at a Bloomberg conference.

The bill also faces opposition from some conservative Republicans opposed to delegating power to the White House, as well as Democrats worried about the impact on jobs and the environment.

Chuck Schumer, tipped to become the Senate's Democratic leader after the 2016 elections, told a committee hearing he opposes TPA and that it is not fair to rush such an important issue, a point also made by other Democrats on the Senate Finance committee.

"You can't fast track fast track - that's a complete abdication of our responsibilities," said Ohio Democrat Sherrod Brown.

Unions said they would launch an advertising campaign to pressure members of Congress to oppose fast track, starting with digital ads but possibly expanding to TV, radio and newspapers.

But businesses said TPA was vital to secure trade deals that would reduce tariffs for manufacturers and other exporters.

"Manufacturers need TPA and new market-opening trade agreements now more than ever," said David Farr, a spokesman at the National Association of Manufacturers.