Bernie Sanders, the democratic socialist U.S. senator challenging for the 2016 Democratic presidential nomination, urged the race’s presumed frontrunner Hillary Clinton on Sunday to take a stand on the Trans-Pacific Partnership (TPP) — the controversial 12-country trade deal that has divided the Democratic Party.

Sanders, a vocal critic of free trade, called on Clinton to join labor unions, environmentalists and other opponents of the trade package before legislation intended to "fast-track" TPP is brought up for another vote this week.

Clinton aides appearing on Sunday television news shows said she would not weigh in until negotiations were complete. But in a subsequent stump speech in Iowa, the candidate herself made comment on the TPP, if only to urge Obama to work with House Democrats who are skeptical over the deal and to voice concern over “objectionable parts” of the proposed agreement.

The remarks are unlikely to have satisfied Sanders, who has used the issue to drive a wedge between the policies of both candidates, and to undermine Clinton’s claims of a progressive agenda.

"Corporate America and Wall Street are going to bring that bill back," Sanders told CBS's "Face the Nation." "If she joins us, we could stop this disastrous deal once and for all."

Democrats in Congress dealt a blow to President Barack Obama on Friday when they stymied an attempt to pass "fast-track" legislation that would have allowed TPP to be passed without amendments in a straight up-or-down vote.

At a campaign stop in Iowa, Clinton said Obama should work with opponents like House Democratic Leader Nancy Pelosi, who helped engineer Friday’s defeat.

"I am willing to try now to see whether you can push to get rid of the objectionable parts, to drive a harder bargain on some of the other parts," Clinton said.

If Obama does not get the best deal possible, "there should be no deal," said Clinton.

The Trans-Pacific Partnership is shaping up to be a significant test for Clinton as her party has grown more suspicious of the merits of free trade since her husband, Bill Clinton, signed the North American Free Trade Agreement into law as president in 1993.

Clinton has expressed reservations about free trade deals in the past, but she played a central role in trade talks with the 11 countries involved in the TPP as Obama's Secretary of State, a post she stepped down from in 2013.

Clinton campaign chairman John Podesta said she would render a judgment when the deal is final.

"She has a clear standard that it's got to be good for American workers or she thinks the United States should walk away from it," he said on NBC's "Meet the Press."

Reuters