A top ally's prediction that Clinton would flip-flop after the election and support the trade pact -- after escaping the political threat from Bernie Sanders.
Making things more uncomfortable: President Barack Obama's appearance Wednesday night to make the case for Clinton. His administration negotiated the deal; she says she opposes it.
So does her opponent, Donald Trump, who seized on the Democrats' mixed messaging Tuesday to slam Clinton.
Here's what you need to know about the deal -- and why it's a focal point in Philadelphia:
What is the Trans-Pacific Partnership?
It's a 12-nation free trade agreement negotiated by the Obama administration. It's designed to wipe out tariffs with countries like Japan, Australia and Malaysia and expand on the existing U.S. trade deal -- the North American Free Trade Agreement -- with Canada and Mexico.
Obama argues the deal's provisions covering labor rights and environmental protections make it the best trade pact liberals have ever seen. Also backing the deal: traditional Republican allies in the business lobby, including the U.S. Chamber of Commerce and the National Association of Manufacturers.
But it's vehemently opposed by traditional Democratic allies like labor unions, which see it as a threat to manufacturing jobs that weren't already siphoned away by automation, previous trade deals and an increasingly global economy. And Trump has hammered it as a poorly negotiated giveaway of U.S. power.
What's Hillary Clinton's position?
She opposes the deal.
But while serving as Obama's secretary of state, Clinton praised TPP as a deal that "sets the gold standard in trade agreements."
Then, last fall, once the 12 participating nations announced they had finalized the text after years of negotiations, Clinton said she opposes it -- citing its lack of a crackdown on currency manipulation and provisions to extend pharmaceutical drug companies' patent protections in poorer countries.
"I did say, when I was secretary of state, three years ago, that I hoped it would be the gold standard," Clinton said at an October debate with Sanders. "It was just finally negotiated last week, and in looking at it, it didn't meet my standards. My standards for more new, good jobs for Americans, for raising wages for Americans. And I want to make sure that I can look into the eyes of any middle-class American and say, 'This will help raise your wages.' And I concluded I could not."
Clinton's history is mixed on free trade. She spoke in favor of NAFTA when her husband signed it into law, but called it a "mistake" during her 2008 presidential campaign. She voted both for and against trade deals during her eight years in the Senate.
But will she flip?
Virginia Gov. Terry McAuliffe -- a long-time very close friend of the Clintons -- told Politico Tuesday in an interview that Clinton would support the trade deal after the election.
"I worry that if we don't do TPP, at some point China's going to break the rules -- but Hillary understands this," he said. "Once the election's over, and we sit down on trade, people understand a couple things we want to fix on it but going forward we got to build a global economy."
Pressed on whether Clinton would reverse her position, McAuliffe told Politico: "Yes. Listen, she was in support of it. There were specific things in it she wanted fixed."
In just moments, McAuliffe had given Americans reason to doubt Clinton's sincerity on the Trans-Pacific Partnership and suspect she's willing to do anything that's politically expedient. That's a problem for Clinton: Two-thirds of Americans already view her as untrustworthy.
Clinton campaign chairman John Podesta quickly sought to undo the potential damage of McAuliffe's remark, saying on Twitter: "Love Gov. McAuliffe, but he got this one flat wrong. Hillary opposes TPP BEFORE and AFTER the election. Period. Full stop."
What's Donald Trump doing about it?
He's essentially saying, I told you so -- and this is another reason you can't trust Clinton.
Trump's opposition to free trade deals -- particularly NAFTA and the TPP -- are animating causes of his candidacy. He's railed against them at every campaign rally since he kicked off his campaign 13 months ago.
On Twitter, Trump -- who is targeting manufacturing-heavy states like Ohio, Pennsylvania and Michigan as his path to the presidency -- immediately attacked Clinton.
"Just like I have warned from the beginning, Crooked Hillary Clinton will betray you on the TPP," Trump tweeted.
Can things get more awkward?
Yes -- particularly Wednesday night at the DNC, when Obama and Vice President Joe Biden will speak.
The Trans-Pacific Partnership is perhaps the single biggest difference Obama and Clinton have at this stage.
And, like Clinton, her running mate, Virginia Sen. Tim Kaine -- who's also slated to speak Wednesday night -- has supported the TPP in the past, only to say he'll oppose it now.