Democratic presidential front-runner Hillary Clinton continued attacking Republican front-runner Donald Trump on May 5, calling his discourse "hateful" during a rally in California. (Reuters)

Hillary Clinton has signaled that if she is elected president in November she would oppose a vote on the Trans-Pacific Partnership trade accord during a lame-duck session of Congress, sharpening her differences with President Obama as he is ramping up his sales pitch on behalf of the deal.

Clinton, the Democratic pres­idential front-runner, responded in writing to a question on the lame-duck session from a coalition of Oregon labor unions and environmental groups by stating: “I oppose the TPP agreement — and that means before and after the election.”

Opponents of the pact said Clinton’s response on the questionnaire, coming ahead of Oregon’s Democratic primary on May 17, represents a more definitive statement of opposition to the 12-nation Pacific Rim accord than she has given before. It could present new hurdles for the Obama administration, which is viewing a likely brief session of Congress after the Nov. 8 election as its last chance to get the deal ratified by lawmakers before the president leaves office in January.

Clinton supported and promoted the TPP while serving as secretary of state, but she has moved to the left in a hard-fought primary campaign against Sen. Bernie Sanders (Vt.), who has long railed against U.S. trade pacts. Sanders also opposed a lame-duck vote, telling the Oregon Fair Trade coalition: “Holding a vote on the TPP during a ‘lame duck’ session would be going against the will of the people.”

Business leaders, including U.S. Chamber of Commerce President Thomas J. Donohue, have said they believe Clinton will come around and support the TPP if she wins the general election.

Her moves further left could make that more difficult.

“I strongly suspect she would like it done and out of the way” before she takes office, said Philip Levy, a senior fellow at the Chicago Council on Global Affairs who served as a trade adviser on Sen. John McCain’s 2008 presidential campaign. “That would make life easier for her.”

But, Levy added, Clinton’s likely Republican opponent in the general election, business mogul Donald Trump, also has denounced trade pacts, and the issue could prove pivotal in industrial swing states in the Midwest.

“Secretary Clinton had to assume she’d be attacked from the right on this,” Levy said. “You have Trump attacking her from the left. In places like Pennsylvania and Ohio, Trump will be saying, ‘You all know she really wants this deal to pass.’ ”

A Clinton spokesman did not respond to requests for comment.

For Obama, the economic anxiety among 2016 primary voters has made passage of the TPP, one of his top economic priorities, look increasingly doubtful.

On Tuesday, The Washington Post published an op-ed article by Obama in which he stated that he empathized with “the skepticism people have about trade agreements, particularly in communities where the effects of automation and globalization have hit workers and families the hardest.”

But, he continued, “building walls to isolate ourselves from the global economy would only isolate us from the incredible opportunities it provides.”

White House press secretary Josh Earnest said Thursday that administration officials are in contact with Republican leaders in Congress over the timing of a potential vote.

“The political calculation I would acknowledge is complicated. It doesn’t fall cleanly along party lines,” Earnest said. “So we’re going to work in bipartisan fashion to develop a strategy that will lead to success.”

Earnest delivered a warning to pro-trade advocates in Congress that they may be running out of time. He said the prevalent anti-trade views among the major candidates to replace Obama should be “rather compelling to supporters of TPP wondering whether or not Congress should act this year.”

On the Oregon questionnaire, in response to questions about provisions in the TPP addressing human rights and labor conditions, Clinton said: “I’m not interested in tinkering around the margins of our trade policy. I think we need a fundamental rethink of how we approach trade deals going forward.”