President Obama addresses the Summit on Worker Voice at the White House on Wednesday. NICHOLAS KAMM/AFP/Getty Images

Hillary Clinton’s opposition to the Trans-Pacific Partnership trade deal makes President Obama’s job of selling the 12-nation pact to an already skeptical Congress much more daunting. The potential impact is likely to be greatest among the small number of Democrats the president needs to push the deal over the top.

Clinton's decision, announced in an interview with Judy Woodruff on PBS, “gives permission for those who voted for fast-track to go the other way and vote against the deal,” said Robert Borosage, president of the liberal Campaign for America’s Future, referring to the votes earlier this summer on Obama’s request for Trade Promotion Authority before he concluded negotiations on the TPP.

Despite vigorous lobbying by the White House and corporate interests, only 28 Democrats in the House and 13 in the Senate voted for the fast-track measure—just enough for a narrow victory for the administration. Now, with the fight beginning over the actual deal, those 45 Democrats are the prime targets because the White House recognizes there are not enough Republican votes to pave the way to victory.

“This is an agreement that is likely to be ratified only if enough Democrats and Republicans support it,” said White House press secretary Josh Earnest before Clinton made her announcement. “We’re going to need members from both parties. It’s not going to be possible to ratify this trade agreement strictly along party lines.”

Earnest acknowledged that it is a tougher sell for Democrats than for Republicans, who traditionally espouse free-trade sentiments. He said the summer’s debate on fast-track showed that “more Republicans in Congress are receptive to the arguments about the benefits of a trade agreement like this than Democrats.” Because of that, he said, “the president took a rather practical approach to building bipartisan support for Trade Promotion Authority, and he’ll take a similarly practical approach in building bipartisan support for the Trans-Pacific Partnership as well.”

The large progressive coalition fighting Obama on the deal includes labor unions and anti-trade groups, and it is likely to be emboldened by Clinton’s reversal of her many pro-trade statements as secretary of State. Already, those groups have taken aim at the 45 Democrats who heeded Obama’s pleas on the earlier votes. Just a few hours before Clinton made her announcement, “multiple national progressive organizations, including the [Progressive Change Campaign Committee] and a national union,” met in Washington with a Democrat considering a primary challenge next year, according to Adam Green, cofounder of the PCCC, a 950,000-member organization. Green did not disclose the targeted incumbent.

Green characterized Obama’s pitch to wavering Democrats as asking them “to walk the plank by voting for a deal written by big corporations for big corporations with the explicit goal of undermining American laws, workers, communities, and the environment.”

Both Green and Borosage promised a massive mobilization of progressive groups and voters to keep the pressure on until the vote, which is not expected before April.

The White House promises an “aggressive” effort to counter the liberal pressure, promising to “stand with” any Democrat who votes with Obama. That could mean help in any primary challenges with fundraising assistance, presidential visits, and official support. A senior White House official, anticipating the efforts to flip the Democrats who voted for fast-track, stressed Wednesday that they will find the president “standing next to them if they come under assault.”

The official said it is too early to give vote counts. “It could go either way—no predictions. But we have a very strong case to make,” he said, suggesting this battle could be easier than the earlier fight over TPA. This time, he told National Journal, “We have more details and more hard facts. For instance, 18,000 taxes will be cut on business. So members will have a pretty stark choice: Do you support cutting 18,000 taxes, or not?”

Borosage also acknowledged the dynamic will change once the full text of the deal is released. “Once you have the agreement, you have particular corporate interests in particular districts that have benefits that come out of the deal,” he said. “So there is a different calculation just in terms of constituent interest that may weigh in.” He also anticipated more of an effort by the president to change votes in the Congressional Black Caucus. “I was surprised we did so well the first time,” he said.

Helping the progressive campaign with Democrats, Borosage suggested, is the natural ebbing of power from any president entering his final year in office. “It gets harder as you get closer to being done,” he said. “It is stunning the extent to which Obama pulled out the stops on TPA and had such little effect on the Democratic caucus. It shows he was totally out of step with the caucus and the base and he didn’t have the sway to turn them.”

While most of the spotlight will be on the president’s efforts with his fellow Democrats, there are two reasons for the White House to worry about holding the Republican votes it had on TPA. First is the departure of House Speaker John Boehner. No one expects the next speaker to have Boehner’s willingness to twist the arms of conservatives influenced by the tea party. Second, the vote will take place in the heart of the presidential campaign. If Donald Trump, the current front-runner, remains a force in the race, his opposition to TPP could sway GOP members of Congress.

This article has been updated.

Tom DeFrank contributed to this article

We want to hear what you think about this article. Submit a letter to the editor or write to