Sen. Elizabeth Warren (D-Mass.) on Tuesday called on Democratic presidential candidate Hillary Clinton to clarify her position on the controversial Trans-Pacific Partnership (TPP), the corporate-friendly trade deal that has become one of the most contentious issues in Congress and the 2016 presidential campaign.

Without knowing Clinton's stance on the TPP, Warren said in an interview with reporters in Iowa, she could not answer whether she would vote for the former Secretary of State next year.

"She has already said that she is opposed to that, and I would like to see her be clearer on that," Warren said. "Right now I think it’s important for her to have a chance to lay out her views on a whole host of issues, including trade."

"For me this is not personal, for me this is about protecting American workers," she added.

Warren has become one of the most outspoken opponents of the TPP, which critics say puts the environment, public health, and labor rights at risk for the benefit of corporate interests.

The text of the TPP has been kept hidden from the public and negotiated in secret meetings—although some of the language has been disclosed through unauthorized leaks—even as its supporters insist that the deal would be a boon to international trade. Warren has repeatedly called on President Barack Obama to release the text.

On Tuesday, she repeated that demand, saying she would not be able to decide how to vote on the deal without reading it.

"I want to judge the final agreement," she said. "These are the issues I have fought to try to protect for pretty much all of my career, long before I ended up in this place. But I think these are powerful issues that affect our country. And I think it's good to get out there, and debate them, to talk about them, to put them in front of the American people."