Washington (CNN) It was their last, best chance at a big, bipartisan deal: President Barack Obama and congressional Republican leaders all agreed on free trade.

Just a little more than a year ago, that philosophical alignment looked like enough for Obama's signature trade deal and centerpiece of his Asian pivot policy -- the 12-nation Trans-Pacific Partnership -- to clear Congress.

Then the 2016 presidential campaign intervened. Now, as Obama participates in his last Southeast Asian summit to promote the pivot and its massive trade pact, the TPP looks like it's headed to the political graveyard.

Obama tried to sound optimistic Wednesday about the deal's future, while also conveying to leaders in Asia the reality of the obstacles.

"I believe that we'll get it done but it's always going to be hard," Obama said at a news conference in Laos, suggesting the deal's path might be easier after the US election. "Nothing is easy in the US Congress right now. Maybe there was a time when it was but I haven't seen it."

Republican presidential nominee Donald Trump roused a populist base with his strident anti-trade message, declaring the Pacific Rim deal "terrible" and a "rape" of American workers. Democratic nominee Hillary Clinton, seeking to minimize the threat of primary opponent Bernie Sanders, announced that she, too, opposes the TPP. And, sensing the political risk of supporting a controversial deal both presidential nominees oppose, several lawmakers changed their positions.

Trump is expected to call Wednesday for eliminating the sequester on defense spending and bolstering the US' defenses by proposing a "major investment" in US military spending.

As Obama tries to cajole last concessions from partner countries on his final trip to Asia, the same congressional leaders who'd nudged him along on the TPP now say it's unlikely to be ratified.

"The current agreement, the Trans-Pacific agreement, which has some serious flaws, will not be acted upon this year," Senate Majority Leader Mitch McConnell flatly declared at a Kentucky Farm Bureau event last week.

Right now, with even pro-trade stalwarts like McConnell opposed to ratification during Obama's term, advocates of the TPP admit that they don't have the votes necessary to clear the House and the Senate.

The summer has featured a rush of Republicans -- particularly those in competitive races -- bolting from the trade deal.

Sen. Pat Toomey, a Pennsylvania Republican and long-time trade supporter, wrote in August in a Pittsburgh Post-Gazette op-ed : "We should dump the TPP and return to the negotiating table to get an agreement that would create jobs and economic growth here at home."

Missouri Republican Sen. Roy Blunt told reporters he's having second thoughts about the deal . Florida Sen. Marco Rubio, once a Trump challenger, hasn't taken an official position, but he stripped mention of trade -- and his support for earlier steps greasing the wheels for TPP -- from his website, RealClearPolitics recently reported

And Sen. Rob Portman, an Ohio Republican who once negotiated pacts as the US trade representative under George W. Bush, opposes the TPP.

Conservative activists who waged a battle to deny Obama trade promotion authority in the first place, meanwhile, say they believe Trump awakened a party that had been ignoring its loyalists on trade.

"I think this will last. Every time an issue comes to the forefront, I've never seen the activists forget about it. Once that issue is in the quiver and activists are paying attention to it, it stays there and activists will fight it forever," said Tea Party Patriots co-founder Mark Meckler. "The stain is here to stay."

The deal's supporters -- including the US Chamber of Commerce and the National Association of Manufacturers -- aren't giving up. But they acknowledge the uphill battle on Capitol Hill.

Linda Dempsey, the National Association of Manufacturers' vice president of international economic affairs, said that, "additional leadership is needed in Washington to forge a viable path forward."

The US manufacturing sector "continues to lose foreign sales and access to new customers in some of the fastest-growing markets in the world," Dempsey warned about the deal falling through.

Obama and TPP advocates have a problem with the president's typical allies on the left, too.

Only a small group of Democrats -- mostly members of the centrist New Democrat Coalition -- backed trade promotion authority in the first place, making for the unusual coalition of Obama and congressional Republicans.

Since then, those Democrats have been under pressure from labor unions that argue the TPP, which they've dubbed "NAFTA on steroids," would siphon more manufacturing jobs away from the United States.

Sanders stoked opposition to the TPP among unions and progressives during the primaries, making it a mainstay of his stump speech and his go-to answer about what differentiated him from Obama and Clinton.

That forced Clinton -- who'd declared the TPP a "gold standard" trade deal in 2012 while serving as Obama's secretary of state -- to the left. Her announcement in the fall of 2015 that she opposes the deal made Obama's task of getting Democrats on board even tougher.

Opponents, meanwhile, are declaring victory -- at least for now.

"It won't come up because the votes aren't there," Richard Trumka, the head of the AFL-CIO, a labor union that opposes TPP, said at a Christian Science Monitor Breakfast Thursday. "The candidates running will have to declare where they stand on TPP and the chips will fall where they may."

Republican House Speaker Paul Ryan has maintained the deal would need substantial renegotiation -- a tough task since each of the other 11 participating nations have their own political sensitivities and Obama has less than five months remaining in office.

"They have to fix this agreement and renegotiate some pieces of it if they have any hope or chance of passing it," Ryan said in an early August interview with Wisconsin Public Radio. "I don't see how they'll ever get the votes for it."

But McConnell made clear last week that the path to the TPP's passage isn't totally closed.

"It will still be around. It can be massaged, changed, worked on during the next administration," he said.

Even if Congress ultimately does approve the Trans-Pacific Partnership, the next president will have ways of blocking its implementation -- such as procedural steps like refusing to officially notify other countries of the United States' implementation of the deal or not verifying that those countries have satisfied their TPP commitments.

The hope of trade supporters is that a President Clinton or Trump would drop their resistance to the trade deal once in office. If they did, they'd be following a path similar to Obama, who pledged as a presidential candidate to renegotiate the North American Free Trade Agreement but governed as a trade supporter who entered new negotiations with the European Union and Asia-Pacific countries.

But trade was less of a driving issue in the 2008 race, and neither Clinton nor Trump has left much wiggle room for a post-election reversal.

"I will stop any trade deal that kills jobs or holds down wages -- including the Trans-Pacific Partnership," Clinton said in August in Warren, Michigan. "I oppose it now, I'll oppose it after the election, and I'll oppose it as president."

Trump -- whose strident rejection of the TPP and other trade deals is a staple of his campaign events -- denounced the pact again on conservative radio host Laura Ingraham's show Thursday.

"It's a terrible deal for the United States," he said. "It's a terrible deal for our workers."