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Hillary Rodham Clinton dealt a significant blow to President Obama in his efforts to secure approval from Congress on his signature trade agreement, saying on Wednesday she could not support the Trans-Pacific Partnership, the 12-nation trade pact that she bolstered as secretary of state and that liberals in the Democratic Party have vehemently opposed.

After months of delicately avoiding expressing an opinion on the controversial trade deal, Mrs. Clinton said the agreement in its current form did not meet her high bar for protecting American workers, the environment and advancing national security.

Her opposition to the trade pact comes just before next Tuesday’s first Democratic presidential debate and represents the latest and most potentially damaging break with Mr. Obama.

In recent weeks, Mrs. Clinton has taken a series of stands important to the liberal wing of the party, which has been increasingly swept up by the insurgent candidacy of Senator Bernie Sanders. Last week, she proposed doing away with the so-called Cadillac tax on certain health care plans, aligning herself with labor unions on undoing a key part of President Obama’s Affordable Care Act. Last month, she came out against the Keystone XL pipeline, which the administration has not yet decided on.

But while Mrs. Clinton’s opposition to the trade pact could do much to appease Democratic voters and labor unions that have seized on the deal as a symbol for the perils of globalization, her decision to repudiate a major legislative goal of Mr. Obama’s — one she initially supported — carries significant political risks.

In a statement, Mrs. Clinton said she was still studying the details of the pact — the largest regional trade deal in history and aimed at bringing together nations representing two-fifths of the global economy.

“But based on what I know so far, I can’t support this agreement,” she wrote. “The bar here is very high and, based on what I have seen, I don’t believe this agreement has met it.”

As soon as Mrs. Clinton made her opinion known her opponents seized on her apparent inconsistency on the issue.

In a 2012 speech in Australia, Mrs. Clinton had said the trade pact, which would bring together the economies of countries from Chile to Canada to Japan, “sets the gold standard in trade agreements to open, free, transparent, fair trade, the kind of environment that has the rule of law and a level playing field.”

“Wow! That’s a reversal!” said Martin O’Malley, a former governor of Maryland who is also seeking the Democratic nomination. Reince Priebus, the chairman of the Republican National Committee, called Mrs. Clinton’s position “a case study in political expediency.”

And, as Vice President Joseph R. Biden Jr. contemplates his own run for the presidency, Mrs. Clinton risks alienating Mr. Obama’s coalition of African-American supporters, partly drawn to her candidacy because of her loyal service in the Obama administration.

Mrs. Clinton and Mr. Biden have similar records on many issues, but by rejecting policies like the trade pact, Mrs. Clinton can draw a distinction with him in anticipation a potential challenge.

Mr. Biden said in a statement Wednesday that he supported the deal and would help Mr. Obama pass it in Congress. But during an event with organized labor at the White House, he stressed the need for greater collective bargaining to help workers. “Folks in the other team argue it’s all about globalization,” he said.

On Wednesday, hours before Mrs. Clinton first made her opinion known in an interview with PBS, the White House press secretary, Josh Earnest, hinted at displeasure inside the West Wing about Democratic opposition to Mr. Obama. He said he hoped Democrats would “remain open to the strong case we have to make.”

Now that Mrs. Clinton has made her views known, Mr. Obama is in the awkward position of pushing a deal that could cement his economic and foreign policy legacy and that he says would “rebalance” relations between the United States and eastern Asia as his former secretary of state campaigns against it.

Because of late concessions to the pact, Mr. Obama has likely lost the support of some key Republicans in Congress, putting added pressure on him to appeal to Democrats who may be less inclined to get behind the deal now that the party’s most prominent presidential candidate has spoken out against it.

“I appreciate the hard work that President Obama and his team put into this process and recognize the strides they made,” Mrs. Clinton wrote in the statement.

She specifically criticized the agreement for lacking sufficient protections against currency manipulation, which she said “kills American jobs,” and provisions that benefit global pharmaceutical companies over patients.

But the current version of the pact is far more attuned to these issues than it was in 2012, when Mrs. Clinton supported the deal.

After the last two rounds of negotiations, in Atlanta and in Maui, concessions were made to cut back on the number of years drug companies could receive intellectual property protection and to ensure that tobacco companies could not use the agreement’s adjudication procedures to sue countries over their antismoking efforts.

On Wednesday, the White House released a report citing the economic benefits of the trade agreement, saying it would eliminate 18,000 taxes that other countries impose on the sale of goods produced in America.

Mrs. Clinton’s adroitly worded statement appeared to leave an opening for her to change her mind, should the deal continue to morph.

“I still believe in the goal of a strong and fair trade agreement in the Pacific as part of a broader strategy both at home and abroad, as I did when I was secretary of state,” she wrote.

The cautious rejection of the deal had echoes of Mrs. Clinton’s complicated approach to the North American Free Trade Agreement, which former President Bill Clinton signed into law in 1993 and which still enrages labor unions and liberals.

In the 2008 Democratic contest, Mrs. Clinton tried to gingerly oppose Nafta, a central piece of her husband’s economic legacy. “The fact is, she was saying great things about Nafta until she was running for president,” Mr. Obama said of Mrs. Clinton during their 2008 nominating fight.