Getting Congress to bless the pact before Obama leaves office will require a high-wire act from the President. He’ll need Senate Majority Leader Mitch McConnell and newly minted House Speaker Paul Ryan to put together large numbers of Republican votes—not to mention a big push from the U.S. Chamber of Commerce and much of the business community.

Earlier this year, the Republicans came together with enough moderate Democrats to give Obama fast-track authority to negotiate the deal, with a 218-208 vote in the House and a 60-38 vote in the Senate. That sets up votes on the TPP—without the right to filibuster or add amendments—for sometime next year.

For now, Ryan and McConnell are reviewing the agreement, and if either of them gives it the thumbs down, the deal will be effectively dead for the rest of Obama’s presidency. The early expectation on the Hill is that the deal should be able to squeak through the Senate, but its chances in the House are anyone’s guess.

Indeed, the deal has run into tough sledding in recent months, despite a full-court press from the White House and the President selling the lowered tariffs on 18,000 U.S. exports as GOP-friendly tax cuts.

Here are the TPP’s key hurdles:

Clinton. The former secretary of State praised the emerging pact as the “gold standard” of trade deals while she was Secretary of State. But last month she slammed the final agreement as she tried to face down a surging challenge from Sen. Bernie Sanders in the Democratic presidential primary. Her opposition to the bill—albeit mushy compared to Sanders’ full-throated denunciations—makes it a tougher sell for fence-sitting Democrats who are already under intense pressure from unions and environmental groups.

Trump & Co. Frontrunner Donald J. Trump has savaged the deal in part because it doesn’t include tough enforcement provisions to fight currency manipulation—the same issue Clinton cites. Other Republican candidates are divided. Whoever is winning when the votes on TPP are taken—no earlier than March—could impact the outcome. A senior House Republican aide said the White House will need to find more Democratic supporters than the 28 who backed TPA. “Given the Trump buzz, those extra Ds will certainly be important,” the aide said.

Obamatrade. Obama’s name on a trade pact automatically makes it harder for Republicans to support. A host of right-leaning websites have kept up a drumbeat against the deal. One group to watch: Heritage Action. Fast-track negotiating authority, also known as Trade Promotion Authority (TPA), narrowly passed the House earlier this year despite the group’s opposition, which was related to concerns about the Export-Import Bank. Heritage, however, has generally favored the idea of free trade, so if it and other groups on the right support TPP, that could provide a cushion.

Tobacco. Yes, cigarettes are dangerous to the trade pact’s health. Here’s how: the deal allows countries to exempt tobacco imports from the new rules. As a result, the trade deal has already lost votes. North Carolina Senators Thom Tillis and Richard Burr backed TPA but now hope to sink TPP in large part because of the tobacco exemption. And McConnell months ago declared it was “essential” that American tobacco exports have the same protections as other goods, citing tobacco farmers in Kentucky.

Intellectual Property. Senate Finance Chairman Orrin Hatch last week said TPP may need to be renegotiated to better benefit the pharmaceutical industry’s exports. That’s something the White House has rejected. But losing the powerful Utah Republican would be a big blow.

The Clock. Eight years ago, George W. Bush tried to get a Democratic Congress to approve three free trade agreements—Colombia, South Korea and Panama—late in his second term. Then-Speaker Nancy Pelosi, D-Calif., effectively stuck them all in a drawer. They only became law in 2011 under Obama and a divided Congress. Obama is now in Bush’s position: Republicans hoping for a better deal—or simply wanting to deny Obama a win—could be tempted to kick the can until a new president takes office.

No matter what happens, don’t expect the White House and congressional leaders to move toward a vote unless they are confident of victory. “As they say, you go to the floor when you have the votes,” the House Republican aide said.

One scenario getting a lot of talk on Capitol Hill is the idea of delaying a final vote until the lame duck session after the elections. Lame-duck lawmaking has been a hallmark of recent Congresses, but it’s also the kind of thing that tends to infuriate members of the House Freedom Caucus, who made life miserable for Ryan’s predecessor, John Boehner of Ohio.

Sen. Sherrod Brown, an Ohio Democrat hoping to kill TPP, said the politics of it are bad and that’s why it might get kicked to a lame duck, or later.

“Of course they don’t want it as part of a presidential campaign…No surprise,” he said.

Brown said he expects that Hatch and McConnell, both longtime trade deal backers, will ultimately fall in line. “There is no way they’re going to vote no,” he said.

Of course, House leader Ryan might be loath to try a lame-duck maneuver with another speakership election coming in January 2017. But if an anti-TPP Democrat wins the presidential election and Congress hasn’t yet approved the deal by then, there could be a now-or-never push to get it across the finish line.