One of Barack Obama’s final acts in office could be to rain on Hillary Clinton’s political honeymoon, if she wins the election in November.

The Trans-Pacific Partnership, a massive trade deal with a dozen pacific rim countries, is one of the last things on Obama’s to-do list. The president is expected to bring it up before Congress during the post-election lame duck session in the hopes of getting it approved before he leaves office.

A lame-duck fight on the TPP would put potential president-elect Clinton in an awkward position. On one side would be her ally Obama and the other the left-wing of her party, which already distrusts her commitment to their issues.

Related: Clinton Running the Table in Key Battlegrounds

Clinton opposes the TPP, which she came out against under pressure from her left flank, despite promoting it as secretary of state.

“I oppose it now, I'll oppose it after the election, and I'll oppose it as president,” Clinton said in a recent economic speech.

But it remains to be seen how vocal her opposition will be during a lame duck fight, when many Democrats in Congress may be looking to her for guidance.

That’s why progressives are already gearing up to push Clinton. On Thursday, a coalition of liberal groups plan to deliver a letter, signed by tens of thousands supporters, asking Clinton to come out against any vote on TPP during the lame duck session of Congress.

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The letter, which was obtained first by NBC News, is signed by the Communications Workers of America union, the online organizing groups CREDO, Democracy for America, Demand Progress and Fight for the Future, the environmental groups 350.org and Friends of the Earth, and the consumer watchdog Public Citizen. Many of the groups supported Sen. Bernie Sanders in the Democratic primary.

Related: 5 Things to Know About the Trans Pacific Partnership

“We welcome your past remarks opposing the TPP. In particular, we were greatly encouraged by your words in a recent major economic address,” the letter reads. “Your continued leadership is sadly necessary as President Obama refuses to abandon this unpopular deal.”

The letter goes on to say a lame duck vote is “a tacit admission that corporate interests matter more than the will of the people” because members of Congress are “less accountable to voters” during such a session.

“It is now more important than ever that the country, the Congress, and President Obama hear specifically that you oppose a vote on the job-killing TPP after the election. We urge you to immediately make a clear and strong public statement against holding a vote on the TPP during the ‘lame duck’ session of the 114th Congress,” the letter concludes.

The groups are also promoting a video that highlights Clinton’s own words opposing the TPP. The video is backed by a micro-targeted digital ad buy aimed at Clinton campaign staffers in her Brooklyn headquarters, White House staff, congressional staff, Department of Commerce staff, and other similar audiences.

Sanders, meanwhile, used the launch of his new group Wednesday night to say he too was spoiling for a fight.

“Let me be very clear. I will do everything that I can to defeat the TPP if it comes to the floor of the Senate, as I expect that it will," Sanders said. "The American people have got to stand together and say no to the TPP.”

Clinton spoke favorably about the trade pact as secretary of state and has shown some deference to the White House on issue, even after breaking with the president. For instance, her delegates to the Democratic Platform Committee voted down amendments to add opposition to TPP to the party platform, and instead put forward compromise language more amenable to the White House.

Related: In Biggest Break With Obama Yet, Clinton Opposes TPP Trade Deal

Despite the vocal opposition to the TPP this year from Sanders, Trump and Clinton, a recent Pew survey found voters still narrowly favor the free trade pact.

That’s especially true among people who plan to vote for Clinton in November, who favor the TPP by a wide 59-32 percent margin. Just over half say it will be good for the country. Much of the support seems to stem from Obama’s overwhelming popularity among Democrats, and his support for the deal.

Democrats who favor TPP are cautiously optimistic about the trade deal’s prospects.

A bill to give Obama Trade Promotion Authority passed with some cushion in both chambers of Congress last year, and the threshold for TPP is even lower. Thanks to the approval of the Trade Promotion Authority act, also known as fast-track, Congress has to give TPP a straight up-or-down vote, without amendments, and it can’t be filibustered.

The removal of the filibuster means TPP could lose up to nine or 10 votes in the Senate compared to fast-track and still pass. In the House, fast-rack passed with a 10-vote margin, which is a relatively much more narrow.

So far, most of the Democrats who supported fast-track seem to be holding in favor of TPP.