By Colleen McCain Nelson And William Mauldin 

WASHINGTON--President Barack Obama

defended his push Friday to strike a major trade deal with 11 Pacific nations as necessary to boost the economy and help working Americans, but acknowledged a tough battle ahead to win support from many within his own party.

After long relying on Democrats to advance his agenda, Mr. Obama must now depend largely on Republicans to pass a bill that would clear the way for a trade pact with Japan, Canada, Vietnam and other Pacific nations.

The proposed Trans-Pacific Partnership, or TPP, a major ambition of the president's remaining time in office, pits him against many of the same Democratic lawmakers, labor unions, environmentalists and liberal activists who helped elect him.

Mr. Obama moved to assure critics on the left that the deal would contain provisions protecting human rights and labor and environmental standards. But he gave no ground on his pursuit of the Pacific pact and the so-called fast-track legislation that would ease its passage.

"The current situation is not working for us," he said in extended remarks to reporters, arguing the U.S. already is at a disadvantage in the global economy, and that companies seeking low-cost labor have left the country.

Opposing the deal, he said, meant "ratification of the status quo, where a lot of folks are selling here, but we're not selling there."

The White House already has launched a quiet but sustained lobbying effort aimed at persuading enough Democrats to join Republicans in backing legislation that would allow Congress to vote on trade pacts but not to amend the deals.

With debate now under way in Congress, Mr. Obama is gearing up to make a more aggressive, personal pitch in the coming weeks. In addition to reaching out to lawmakers, the president will make the case directly to opponents in his own party through public events around the country and through what a White House official described as "targeted media engagements."

Mr. Obama's comments came a day after key congressional leaders struck a deal on fast-track legislation and as Democrats continued to voice serious reservations about signing off on the trade-promotion authority that several past presidents have had. Some of Mr. Obama's reliable allies, including Sen. Charles Schumer (D., N.Y.), already have taken a stand against the president's trade ambitions as currently proposed.

"I don't believe in these agreements anymore," Mr. Schumer said this week.

Highlighting the challenge ahead, Mr. Obama won no overt support Friday from Hillary Clinton, the leading Democrat aiming to succeed him in the White House after 2016. In a statement, Her campaign spokesman said the U.S. should be "willing to walk away" from any deal that fails to protect U.S. workers, raise wages, create more jobs at home and enhance national security. The statement gave no verdict on the Pacific deal.

Mr. Obama was unbowed Friday by his party's opposition and by suggestions that his position on trade would damage the party, saying other trade deals have been passed during his presidency, and "it didn't divide the Democratic Party."

He suggested that many Democrats oppose trade because "because the unions on principle, regardless of what the provisions are, are opposed to trade."

Labor unions and other liberal groups have said that the proposed trade deal would hurt U.S. workers and could lead to outsourcing and job losses.

Mr. Obama has promised to avoid the mistakes of past trade agreements. On Friday, he said that once a deal is reached, he would be able to show that it will be good for American businesses and workers.

The administration's efforts to claw back support from some unions and Democratic lawmakers "isn't going to work," said Rep. Rosa DeLauro (D., Conn.), who is spearheading opposition to fast-track legislation in the House. "It is a very formidable united front on this issue that I believe will be maintained."

Fast track is also opposed by some tea-party and conservative groups, including the American Family Association, which represents the religious right. That sets up an unusual bloc of opposition, forcing Mr. Obama to align with a large group of more moderate Republicans in Congress and a smaller number of Democrats.

Unions are particularly upset that the White House is reaching out to local union affiliates in areas where the administration thinks it could bring Democratic members to support fast track.

"So far, I don't think they're getting any positive response on that, and I suspect probably it's backfiring," said Thea Lee, deputy chief of staff at the AFL-CIO, the largest U.S. labor confederation.

Mr. Obama on Friday repeated a recent argument that the TPP won't hurt workers because the pangs of globalization have mostly run their course, and there will be labor and environmental safeguards that prevent job losses blamed on the Nafta deal with Mexico and Canada.

But many Democrats are worried most about U.S. workers competing with low-wage competitors in Vietnam and Malaysia, and unions say they are opposing fast track because they don't trust the administration to enact really tough rules on labor in the TPP.

"The worst thing that the White House says is 'This is how we fix Nafta, ' " said Larry Cohen, president of the Communications Workers of America, a union. "There is no enforceability at all for the language."