If Democrats are successful in November in taking at least a slim majority in the House of Representatives, as most current polls predict, the new NAFTA agreement reached on Sunday night will face an uphill battle to becoming law.

A completed three-way deal won't necessarily be dead on arrival in a Democratic-controlled Congress — but it won't be a foregone conclusion that lawmakers would pass the agreement, either.

Because of several procedural steps that must be followed, a floor vote is not expected until the next Congress is sworn in. That sets the stage for a nail-biting 2019, with the prospect of a protracted showdown between the White House and Capitol Hill.

"It'll be too much fun for them to just kill it off on Day One," said Bill Reinsch, who worked for two decades as a Senate aide.

A flat-out rejection by either chamber would rob President Donald Trump — at least for awhile — of what he would like to be a signature trade-policy accomplishment. A tense back-and-forth would also leave Mexican and Canadian government officials watching nervously on the sidelines, fearing that Trump may carry through on his ultimate threat: to cancel the old NAFTA as a pressure tactic, in his battle against Congress.

It's not outside the realm of possibility. Newly powerful Democrats who despise the president, and who would be loath to hand anything even closely resembling a policy victory to the White House, could simply choose to reject anything that comes their way

Trump himself acknowledged on Monday the political hurdles the administration must overcome in order for the new agreement to be passed, and he admitted he was "not at all confident" Congress would approve it.

"Anything you submit to Congress is trouble," he said.

Speaking from the White House Rose Garden, Trump also suggested that Democrats could be willing to scuttle the deal for purely political purposes, noting "they'll have 2020 in mind."

"Their whole campaign is 'Resist,'" he said. "They don't even know what they're resisting."

A bevy of factors are affecting lawmakers' thinking about how to vote on NAFTA, most of which have less to do with maintaining good relations with Canada and Mexico and more to do with navigating the politics of the Trump presidency or their own districts' economic needs.

Even with the three-way discussions concluded, some could still find structural reasons to vote against the pact. One example would be if members of Congress felt that administration officials did not adequately follow fast-track legislation throughout the negotiating process.

The fast-track law permits the new deal to go through Congress with only a simple up-or-down vote, without lawmakers attaching amendments that could muddle the agreed-to language. But it also requires that the Trump administration keep lawmakers informed of progress throughout the negotiations. Some Democrats have complained since talks began last August that they felt as if they had been left out.

Rep. Bill Pascrell (D-N.J.), ranking member on the House Ways and Means Trade Subcommittee, issued yet another request for a hearing with administration officials in late August via a letter to Rep. Kevin Brady (R-Texas), the panel's chairman. Pascrell called the lack of communication thus far "a failure by the administration" and "a failure by Congress to assert its rightful, constitutional authority in trade negotiations," and warned that officials may have run afoul of fast-track rules.

You don't have to go back to your constituents and argue why it wasn't a good agreement," he said. "You can go back and argue they didn't follow the rules."

Not all Democrats have an interest in rejecting NAFTA, and some pro-free-traders will be keen to shepherd an agreement through. They could unleash some of their own stipulations, however, and use the leverage of their votes in support of the agreement to push the White House into making certain changes.

"They'd want to extract something that they can claim as their own, which I think would be, frankly, only natural," said Welles Orr, a former assistant U.S. trade representative for congressional affairs who now works at the Washington firm Miller & Chevalier. "Democrats would want to revisit certain things ... maybe not want to open up the agreement, but they'd want to take a good long time looking at it."

Of particular interest to Democrats will be the labor and environmental standards that make it into the final deal. The party has for years been calling for those provisions to be strengthened and made easier to apply. Senate Minority Leader Chuck Schumer appeared cautiously upbeat Monday that the new deal could win his support, so long as it includes enforceable labor standards as well as strong dairy provisions.

If a final agreement is signed by all three countries, I also look forward to working with my colleagues in Congress to write 'implementing legislation' to ensure the deal actually achieves these goals," he said in a statement.

Early reaction to the new agreement from labor groups, whose support will be crucial for many Democrats, suggests that they still have concerns about certain provisions even though the Trump administration did make progress on its priorities.

"Added protections for working people and some reductions in special privileges for global companies is a good start, but we still don't know whether this new deal will reverse the outsourcing incentives present in the original NAFTA," said AFL-CIO president Richard Trumka, who leads the largest U.S. labor group. "It also is critical that we see what final labor enforcement, auto rules of origin and government purchasing provisions will look like."

Some Democrats themselves also expressed similar concerns, with Neal for one saying he would be looking at “the enforcement and enforceability of the agreement's provisions." Sen. Ron Wyden (D-Ore.) — the top Democrat on the Senate Finance Committee, who would take over leadership of the panel if control of his chamber changes hands — echoed the same sentiment.

Sen. Sherrod Brown (D-Ohio), another prominent Democrat, has long warned that the current deal is falling short on labor issues. But if the new agreement marks a significant step forward on areas like workers' rights and wages — regardless of whether it goes as far as he and others might like — they might vote in favor of the deal in order to show support for the fact that the new standards were included at all.

Brown also appeared to be reserving judgment of the new deal on Monday, saying his “No. 1 priority” is to prevent the offshoring of Ohio jobs, “and that’s what I’ll be looking for as I carefully review the text.”

If lawmakers try to slow-walk or block the renegotiated deal once it gets to Congress for whatever reason, Trump administration officials have been considering a strategy of withdrawing from the original NAFTA as soon as it sends the 2.0 version to Capitol Hill. Such an aggressive tactic would force lawmakers to choose between Trump's version of the deal or none at all.

That idea has been simmering for months, and the president renewed expectations that he would pursue such a move in late August when he announced the U.S.-Mexico agreement and declared he would be "terminating the existing deal and going into this deal."

While it could pressure lawmakers to approve the new pact, the view among many trade experts is that it could just as easily backfire. Republicans and Democrats alike have warned the administration not to make such a move. And as if trying to preempt the White House from moving forward with their plans, lawmakers have also begun to lay groundwork to block the administration from unilaterally being able to withdraw on its own.

Wyden revived that threat last month when he blasted Trump's trade negotiations as "tissue-paper soft."

The president needs to take a look at the Constitution — it gives Congress authority over trade," he said in a statement at the time. "The president cannot pull America out of NAFTA without Congress' permission."