Sen. Ron Wyden (D-Ore.) has become the White House’s most important ally as President Obama races to complete a massive 12-nation trade deal in the Asia Pacific that is important to his foreign policy legacy. (Victor J. Blue/Bloomberg News)

Three weeks ago, Sen. Ron Wyden got an unlikely phone call. On the line was President Obama, who last year helped foil the Oregon Democrat’s plans to shepherd a modest tax bill through Congress.

Obama was calling to thank Wyden for his support of the president’s sweeping trade agenda which, in a reflection of the topsy-turvy politics of the issue, has drawn support from Republican leaders and staunch objections from many Democrats.

Enter Wyden, 65, a four-term senator who has consistently supported trade. He has become the White House’s most important ally to woo skeptical members of Obama’s own party as the president races to complete a massive 12-nation trade deal in the Asia Pacific that is important to his foreign policy legacy.

“He called to express appreciation with my work,” Wyden said in an interview Wednesday. “The president and I have talked a number of times about this. We are very much committed getting trade done right.”

The personal touch from Obama is one signal of how much a White House not known for its courtship of Capitol Hill is counting on Wyden to help deliver on the trade deal, which the president and his GOP adversaries have called the most likely bipartisan breakthrough in Obama’s final two years.

So far, however, the results have not inspired confidence among Republicans. This week, Senate Finance Committee Chairman Orrin G. Hatch (R-Utah) told reporters that his plans to introduce legislation to grant the administration additional powers to finalize the Trans-Pacific Partnership (TPP) — as well as another large deal between the United States and Europe — would be delayed until April.

It was a setback for Obama. Administration officials have said they need the expanded trade authority soon if they are to meet their goal of completing the TPP deal by the end of the year, before the 2016 presidential politics put a deal beyond reach.

Behind the scenes, Hatch and Wyden, the committee’s ranking Democrat, have been deadlocked over the terms of the “trade promotion authority” legislation — which aims to limit Congress’s authority to amend a final trade deal before voting on it. Wyden is demanding that the bill include additional provisions for lawmakers to scuttle the entire process if the administration fails to meet specific thresholds.

“What some Democrats want would strip away the very integrity of what [trade promotion authority] has stood for — a sense of certainty” among the countries negotiating the deal, Hatch said. “That sets a dangerous precedent.”

Wyden, who faces reelection next year, insisted he is a firm believer in the power of free trade to create jobs and raise wages, as long as the deals have strong and enforceable labor and environmental protections.

As a congressman in 1993, he voted in favor of the North American Free Trade Agreement. In 2002, he was among a minority of Senate Democrats who voted to grant President George W. Bush the unabridged fast-track trade powers. (Those powers expired in 2007, but Obama was able to use them to complete trade deals with Korea, Panama and Colombia whose negotiations had started under Bush.)

But Wyden is facing growing pressure at home from labor unions, and the state’s junior senator, Jeff Merkley (D), has been largely skeptical of trade deals. Wyden said his additional demands from Hatch are aimed at answering concerns from fellow Democrats for more public accountability in trade agreements.

“If you listen to citizens, in public opinion polls and town-hall meetings, there’s a lot of skepticism about the secrecy” over the TPP’s details, Wyden said. “What I’m trying to do is show there’s a lot more transparency and openness.”

Obama, who has sought to shift U.S. foreign policy attention to Asia, has cast the TPP pact as a hedge against China’s growing economic clout. The deal aims to lower tariffs, establish a new regulatory framework to mitigate disputes between member nations and multinational corporations, and create new labor and environmental standards.

Obama and Wyden “share a view that TPP can set historic new standards, particularly as it relates to workers’ rights and the environment, that could serve as a model for future trade negotiations,” White House spokeswoman Brandi Hoffine said.

Though talks have continued — House Ways and Means Committee Chairman Paul Ryan (R-Wis.) is also involved — Hatch is growing increasingly frustrated with his would-be partner. A year ago, Hatch struck a deal with then-Finance Committee Chairman Max Baucus (D-Mont.) and former congressman Dave Camp (R-Mich.) on a bill to give Obama the expanded trade powers.

But when Baucus was tapped by Obama as ambassador to China, Wyden replaced him as chairman and scuttled the agreement, saying he wanted to start from scratch. Though then-Senate Majority Leader Harry M. Reid (D-Nev.) flatly stated that he would not support the fast-track trade legislation in an election year, Republicans have cited the process as evidence that Wyden is wishy-washy on trade.

The White House is standing by him. In recent weeks, administration aides, including Obama adviser Brian Deese, have been in regular contact with Wyden aides to collaborate on strategy, according to Hill Democrats.

Last week, Obama made a public push on trade, promoting his plans in interviews with local television stations, including one on Wyden’s home turf in Portland.

“I’ve made clear how important it is that the president make the case,” Wyden said. “He’s doing that. The only way I can describe my work with the White House is that it has been very constructive.”