By JONATHAN WEISMANAPRIL 16, 2015

WASHINGTON — The leaders of Congress’s tax-writing committees reached agreement Thursday on legislation to give President Obama “fast track” authority to negotiate an ambitious trade accord with 11 other Pacific nations, beginning what is sure to be one of the toughest legislative battles of his last 19 months in office.

The “trade promotion authority” bill — likely to be unveiled Thursday afternoon — would give Congress the power to vote on the Trans-Pacific Partnership once it is completed, but would deny lawmakers the chance to amend what would be the largest trade deal since the North American Free Trade Agreement.

Senator Orrin G. Hatch, chairman of the Senate Finance Committee, and Representative Paul D. Ryan of Wisconsin, chairman of the Ways and Means Committee, had to agree to stringent requirements for the trade deal to win over Senator Ron Wyden of Oregon, the ranking Democrat on the finance panel. Those requirements included a human-rights negotiating objective that has never existed in trade agreements, according to lawmakers involved in the talks.

The legislation would also make any final trade agreement public for 60 days before the president signs it, and up to four months before Congress votes. If the agreement, negotiated by the United States Trade Representative, fails to meet the objectives laid out by Congress — on labor, environmental and human rights standards — a 60-vote majority in the Senate could shut off “fast track” trade rules and open the deal to amendments.

“We got assurances that U.S.T.R. and the president will be negotiating within the parameters defined by Congress,” said Representative Dave Reichert, Republican of Washington and a senior member of the Ways and Means Committee. Referring to the trade promotion authority, he added, “And if those parameters are somehow or in some way violated during the negotiations, if we get a product that’s not adhering to the T.P.A. agreement, then we have switches where we can cut it off.”

Mr. Reichert said the Ways and Means Committee would formally draft the legislation next week.

Even with those concessions, the fight to get the trade promotion bill to the president’s desk will be difficult and emotional, badly dividing the Democratic Party’s labor base and putting Hillary Rodham Clinton in a quandary. Many prominent Democrats have come out against one of the biggest priorities of their president. Representative Sander M. Levin of Michigan, the ranking Democrat on the House Ways and Means Committee, was notably absent from trade negotiations.

The A.F.L.-C.I.O. and virtually every major union have vowed a fierce fight, including a six-figure advertising campaign to pressure 16 selected senators and 36 House members to oppose trade promotion authority, the A.F.L.-C.I.O. announced on Thursday.

“We can’t afford to pass fast track, which would lead to more lost jobs and lower wages,” said Richard Trumka, the A.F.L.-C.I.O. president. We want Congress to keep its leverage over trade negotiations — not rubber stamp a deal that delivers profits for global corporations, but not good jobs for working people.”

Republican leadership is firmly behind the trade authority bill, an exception to the otherwise divisive relationship between Mr. Obama and congressional Republicans. But a sizable minority of Republicans — especially in the House — are reluctant to cede almost any type of authority to the president. Whether Republican leaders can get their troops in line, and how Mr. Obama can round up enough Democratic votes, is emerging as one of the larger legislative questions of the year.

Mr. Reichert said that only 15 to 20 Democrats so far are supportive in the House. Last year, House Speaker John A. Boehner of Ohio said he would need 50.

At a finance committee hearing Thursday morning, Treasury Secretary Jacob J. Lew and Michael Froman, the trade representative, pleaded for “fast track” trade authority.

“T.P.A. sends a strong signal to our trading partners that Congress and the administration speak with one voice to the rest of the world on our priorities,” Mr. Lew testified.

Mr. Hatch called the deal “the most powerful tool in Congress’s trade arsenal” to secure “some of the most ambitious trade agreements in our nation’s history.”

But the fight was laid bare at the hearing. Senator Charles E. Schumer of New York, the third-ranking Democrat, railed against trade agreements that he said he once supported.

Senator Charles E. Grassley, Republican of Iowa, also expressed skepticism when he said, “We have reason to be concerned about overreach from this administration.”