The announcement on Monday that negotiators reached a deal on a sweeping 12-nation Pacific Rim trade deal may have been cause for celebration for President Obama.

But support in Congress is far from guaranteed as the Trans-Pacific Partnership (TPP) agreement is getting a mixed reception from lawmakers.

Due to the rules governing consideration of trade deals, it could be a while before the deal is considered with congressional aides speculating the first votes could start sometime in April. The White House is expected to heavily lobby members in the coming months to try and convince reticent lawmakers in both parties that the deal would boost exports, create new jobs and help make the Pacific region more secure.

[A long road ahead for newly minted Pacific Rim trade agreement]

The president’s biggest advantage in getting congressional approval is that the trade deal will be considered under special fast-track rules, known as Trade Promotion Authority (TPA), that were enacted earlier this year. Under this process, the agreement will be subject to a simple up or down vote and lawmakers can not amend or filibuster the pact.

Working against Obama is the waning support from Republicans for the deal, the expected intense lobbying from outside groups and the continuing skepticism among members of his own party that the agreement will be good for workers.

What follows is a look at the one big reason the deal could get through Congress and the four challenges that could stand in its way.

Obama’s advantage — Fast-track trade rules: Obama worked closely with Republicans in Congress earlier this year to enact legislation that would allow trade deals to be considered under the fast-track TPA rules, a rare area of bipartisan cooperation between the White House and Congress.

Once legislation to implement the pact is introduced, Congress cannot amend the agreement and it takes just a simple majority to approve the deal. Those rules, which are commonly used to help convince international negotiators that Congress won’t meddle in the details of a long-sought agreement, should make it easier to get a bill through Congress. They remove the threat of filibuster and prevent the introduction of any “poison pill” amendments that could sink the entire package.

Due to the long congressional review process established by the law that set up the fast-track rules, it may be April before legislation implementing the deal begins moving.

Between now and then, observers will be keeping a careful eye on how many of the Republicans and moderate Democrats who supported giving the president TPA sour on the actual deal as they pore over the details and get lobbied by special interest groups.

What could go wrong for the White House…

1. Republicans may be hesitant to hand Obama a victory: Republicans have traditionally liked the idea of free-trade agreements, but on TPP some likely backers have already expressed skepticism.

Senate Finance Committee Chairman Orrin Hatch (R-Utah), who was a force behind putting the fast-track rules in place, said in a statement that the TPP falls “woefully short” of the goals and objectives of a successful deal because it does not do enough to improve access to global markets for American-made products.

“The United States should not settle for a mediocre deal that fails to set high-standard trade rules in the Asia-Pacific region for years to come,” Hatch said.

The presidential election could potentially complicate the debate over the trade deal if Republicans become more reluctant to give Obama a victory on an issue he views as a big part of his economic policy legacy.

Not all of the candidates on the GOP campaign trail have opined on the newly-signed TPP, but their positions to date on the issue have been mixed.

Sens. Marco Rubio (R-Fla.) and Ted Cruz (R-Texas), for instance, have spoken positively about the need for such free trade agreements. Rubio voted for the fast-track authority, though Cruz voted against the bill over a dust-up concerning the Export-Import Bank.

Donald Trump, meanwhile, has called TPP “an attack on America’s business.”

2. Obama may not have the support of some key Democrats. It is not uncommon for lawmakers to be cautious in their early assessments of a trade agreement and some key Democrats have been hesitant to back Obama. 

Trade has traditionally been a divisive issue for Democrats who pride themselves on protecting American workers, supporting unions and working for higher wages. But this deal was particularly difficult for some in the president’s party.

“I have said throughout the negotiations that I will oppose any trade agreement that does not protect American workers and human rights, create jobs, and help hard-working families in my district,” said Rep. Linda Sanchez (D-Calif.), a member of the powerful Ways and Means Committee that has jurisdiction over trade. “During the negotiations there was little evidence that this was going to be a good deal for the majority of American workers.”

Some Democrats complained that they were not given the chance to have much input while the deal was being negotiated.

“Early reports indicate that this agreement is a bad deal for the working men and women of the American auto industry and for many other sectors of the U.S. economy,” said Rep. Debbie Dingel (D-Mich.) “The negotiators’ cloak and dagger approach has resulted in Members of Congress and the public being forced to operate on hearsay – not facts – because to date, we have been unable to access the secret web of side agreements and backroom deals that are associated with the latest rounds of negotiations.”

Trade agreements are typically negotiated in private meetings between highly specialized trade negotiators who tell tales of deals hashed out in late night sessions in hotel suites and in tiny basement boardrooms. It is common for elements of a trade agreement to be kept quiet while sensitive details are still in flux, but Democrats grew frustrated with what they saw as an unprecedented level of secrecy in the TPP negotiations.

3. Powerful outside lobbying groups oppose the deal. Special interest groups have been waiting for the details of the pact to emerge and now that lawmakers have the chance to freely delve into the deal’s hundreds of pages of legal jargon the groups will happily be there to help interpret the complicated details.

[Industry, labor and environmental groups gear up to oppose TPP trade deal]

Among the most contentious issues is the way the deal handles pharmaceuticals, particularly a class of drugs known as biologics. Pharmaceutical companies wanted intellectual property protection for those medicines for 12 years, but the trade deal only grants protection for up to eight years. Lobbying group PhRMA said this provision is “disappointing.”

Another contentious issue is the investor-state dispute settlement section, which allows out-of court arbitration between multinational corporations and foreign governments over regulations on such things as  environmental and public health policies. Environmental groups argue this process tends to favor corporations.

The tobacco industry opposes this part of the deal as well, but for a different reason. It is specifically barred from taking advantage of the arbitration process, which will make it harder to challenge anti-smoking initiatives.

4. If the final vote falls during primary season, vulnerable candidates could be nervous about backing a controversial deal.  Congressional aides estimate that the first set of votes on the TPP deal could happen in April, right in the heat of primary season. The first state primaries start on Feb. 1 and many vulnerable members will still be on the ballot in late-spring.

Wisconsin, home to vulnerable Republican Sen. Ron Johnson, has its primary scheduled for April 5. Pennsylvania, home to vulnerable Republican Sen. Pat Toomey, is scheduled to vote on April 26. Though Toomey and Johnson will likely face their biggest challenge in the general election, their votes on trade could become a major campaign issue.

Other senators facing tough reelection campaigns, such as Michael Bennet (D-Colo.) and Kelly Ayotte (R-N.H.), may have early primaries where TPP could be an issue.