(CNN) —  

It was a huge win for President Barack Obama but, as is so often the case in polarized Washington, it may just be a prelude to a bigger battle ahead.

Congress finally granted the president fast-track trade authority on Wednesday, after Majority Leader Mitch McConnell, the Republican who once plotted to deny him reelection, saved a centerpiece of the president’s second term by forcing a bill opposed by most Democrats through the Senate.

The vote revived Obama’s trade agenda, was a lifeline for his signature Asia “pivot” policy – for which the vast trade pact in the works is crucial – and was a rare instance of leaders working across the aisle.

READ: Senate sends key trade bill to Obama

But opponents of the Pacific Rim trade pact, known as the Trans-Pacific Partnership, or TPP, are promising to muster a growing coalition of union, environmental and community groups to heap pressure on lawmakers who will now face a vote on ratifying it in the volatile political atmosphere of the 2016 election campaign.

That’s also bad news for Hillary Clinton.

The front-running Democrat was an architect of the TPP as secretary of state but has taken on the skepticism of many liberals towards big trade deals since she started running for president amid a populist economic storm.

Obama himself fought a ferocious legislative duel that estranged him from some of his closest allies in his own party in order to get it through.

Capitol Hill finds a bipartisan cause

Still, on Thursday, the passage of trade promotion authority – which gives Obama the power to submit trade deals to Congress for an up-or-down vote, seen as essential to getting such pacts approved – was a novel break with Capitol Hill’s habitual inertia.

“It wasn’t easy. Many thought it would never happen,” McConnell said.

White House officials spun the vote as a victory for almost extinct bipartisanship, despite the failure of the bulk of Democrats to sign on and the stunning and uncomfortable showdown between Obama and House Democratic leader Nancy Pelosi over the measure.

White House spokesman Josh Earnest hailed “Republican majorities in Congress working closely with Democratic minorities in Congress to build bipartisan support for legislation that then arrives on the desk of a Democratic president.”

He added, “That is the way the legislative policymaking process in Washington is supposed to work in an era of divided government.”

Privately, there was satisfaction among White House aides that just a few days after many in the media wrote obituaries for Obama’s trade agenda and, not for the first time, proclaimed him a lame duck, he was able to wrack up another big victory for presidential power.

Had the fast-track provision crashed, U.S. partners in the Pacific, including Japan, would never have accepted tough political concessions on market access required by the TPP and the deal would have died, likely taking U.S. credibility in Asia and Obama’s pivot to the region with it.

Now, Obama’s trade negotiators can pursue a final agreement on a deal involving 12 nations accounting for 40 percent of global GDP that would significantly enhance his political legacy.

U.S. President Barack Obama and House Minority Leader Nancy Pelossi walk through a hallway after meeting on trade with House Democrats at the U.S. Capitol on June 12, 2015 in Washington, D.C.
PHOTO: MANDEL NGAN/AFP/Getty Images
U.S. President Barack Obama and House Minority Leader Nancy Pelossi walk through a hallway after meeting on trade with House Democrats at the U.S. Capitol on June 12, 2015 in Washington, D.C.

The deal’s survival also means Obama will be at center stage in Washington fighting for a major legislative priority right until the waning months of his second term, at a time when many presidents have already run out of steam and domestic clout.

He will have little choice but to wage an intense campaign to convince the American people of the TPP’s merits.

Still, final passage of any TPP deal is no sure thing – and not just because Obama will be low on leverage at the end of his mandate.

Vote on trade deal could come in 2016

Supporters of the TPP hoped that they could get Congress to sign off on the pact before the notion of global trade gets caught in the inevitable election-year backlash against Wall Street, corporations and low-wage economies overseas that have syphoned jobs from rust belt swing states in the Midwest and elsewhere.

But now, lawmakers will be asked to take a tough vote that could expose them to attacks from opponents of the deal.

TPP nations may ink a final agreement at a ministerial meeting this summer and could present the deal as the highlight of the Asia Pacific Economic Cooperation (APEC) forum later this year.

But there’s no guarantee the fragile coalition assembled by Obama and McConnell – mostly Republican but bolstered by pro-trade Democrats – will survive. And the clock is ticking.

“I am very doubtful we can have a ratification vote before the end of this year,” said Mireya Solis, a specialist on Japanese economic policy at the Brookings Institution. “We are thinking this is going to spill over to 2016 and this opens up all kinds of interesting possibilities.”

READ: How secretive is the Trans-Pacific Partnership?

Between now and then, Obama must work hard to repair ties with Democrats on Capitol Hill. More mastery of Senate procedure will also likely be required from McConnell, who secured support for the fast-track authority only after promising to adopt a workers’ assistance bill Republicans largely disparaged but Democrats demanded.

The wily Senate leader’s normal antipathy to Obama’s priorities was overcome because Republicans generally still firmly back free trade – and he was keen to use the fast-track vote to convince voters in 2016 that the GOP has delivered results and deserves to keep its slim Senate majority.

“I’ve enjoyed working with the president on this issue,” McConnell said on Tuesday in an incongruous show of warmth, five years after he told National Journal the most important GOP achievement would be “for President Obama to be a one-term president.”

“Hopefully, we can find some other things that we can agree on to work on together during his remaining year-and-a-half in office. “

Few in Washington will be holding their breath, given the scorched-earth policy McConnell has adopted towards Obama and the president’s own clear disdain for Congress as a whole – not just the Republicans who run it.

Peril for Hillary Clinton

While Obama and McConnell can claim a share of the spoils, the big loser could be Hillary Clinton.

The debate over fast-track has put Clinton in a dicey political spot given her past support for the TPP, which she once called the “gold standard” of trade agreements.

Since then Clinton, already facing suspicion among Democratic primary voters about her links to Wall Street and big business, has rowed back – insisting that Congress should use the fast-track authority as leverage to getting a better TPP arrangement.

If a final TPP deal is agreed to, the lengthy ratification process will likely mean Clinton is stuck with the issue through the Democratic primary season.

Wherever she comes down on TPP, she would be open to dreaded charges of flip-flopping by Republicans, which could exacerbate questions of trust in her character that have emerged in recent opinion polls.

And TPP opponents will not make her life any easier.

“We feel emboldened after this fight. We see fast-track as a momentary loss and feel really confident about attacking the TPP head on,” said Ilana Solomon, director of the Sierra Club’s Responsible Trade Program.

“There is going to be an even bigger fight about stopping the TPP,” said Solomon, who like many opponents believes that when the TPP’s secret provisions are revealed when the final agreement is struck by the U.S. and its Pacific partners, it will prove their case that the deal empowers corporations at the expense of American workers.

Some TPP opponents believe the backlash could force the White House and Republicans to delay ratification until a lame-duck session of Congress after the November 2016 election.

And RoseAnn DeMoro, executive director of the National Nurses United union, which also opposes the TPP, predicted that support for the pact could be incompatible with an election in which economic inequality and the power of corporations will be driving issues.

“This is a betrayal at the highest levels. These folks that voted with the President have spoken very clearly,” she said. “You had to choose here. This was pretty black and white. You are with us or against us. Us is the American people. They chose Wall Street.”