In her biggest break with the Obama White House thus far, Hillary Clinton came out against the Trans-Pacific Partnership treaty that the president views as a key legacy item.

“As of today, I am not in favor of what I have learned about it,” Clinton told the PBS NewsHour’s Judy Woodruff in an interview Wednesday afternoon. “I don’t believe it’s going to meet the high bar I have set.”

The massive trade deal with a dozen Pacific Rim nations is one of the most divisive issues inside the Democratic Party, pitting labor unions, some leading Democrats on Capitol Hill and most of the party’s presidential candidates against the White House.

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Clinton had refrained from taking a position on the treaty for months, saying she would wait until international negotiations were complete before reviewing it. Negotiations concluded Monday, but the full text of the treaty has not yet been released. Clinton said she pored over information about the treaty Tuesday night with staff while campaigning in Iowa.

A White House official told MSNBC that Clinton’s staff gave the president’s team a heads up Wednesday, but would not go into more details. “The politics of the trade issue, particularly on the Democratic side of the aisle, are really tough,” White House Press Secretary Josh Earnest noted earlier Wednesday during his daily briefing.

Clinton helped negotiate the deal as Obama’s secretary of state, and once called it the “gold standard” of international trade deals. But it evolved since she left government in early 2013, and Clinton said earlier this summer that she would not support a deal unless it created American jobs, raised wages and advanced U.S. national security.

The Democratic presidential candidate said the TPP did not meet those requirements. She also cited concerns raised by global health groups about pharmaceutical intellectual property and the lack of new enforcement on currency manipulation.

“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, just as I did when I was Secretary of State,” Clinton added in a statement sent to reporters after the interview. “I appreciate the hard work that President Obama and his team put into this process and recognize the strides they made. But the bar here is very high and, based on what I have seen, I don’t believe this agreement has met it.”

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Her opposition to the Obama priority demonstrates how much pressure she is under the Democratic presidential primary, a race in which Sen. Bernie Sanders is putting up unexpectedly strong opposition from Clinton’s left.

Her top rivals for the Democratic nomination, Sanders and former Maryland Gov. Martin O’Malley, both oppose the deal. Vice President Joe Biden, who is considering a presidential bid, would be the only Democratic presidential candidate supporting TPP if he decided to throw his hat in the ring.

Sanders, who fought an earlier congressional trade authorization bill in the Senate, said Clinton’s opposition would have been more helpful months ago when he was fighting that bill.

“I’ll let the American people determine who has credibility or not [on this issue],” Sanders told reporters after a speech to the Congressional Hispanic Caucus Institute in Washington. “I’m glad that she reached that conclusion. This is a conclusion I reached on day one.”

“I can simply say that I am delighted that Secretary Clinton is on-board with opposition to the TPP — to be very frank with you, it would’ve been more helpful to have her on board a few months ago,” he added.

O’Malley wasted no time in exploiting Clinton’s announcement, putting out a statement accusing the former secretary of state of changing her opinion ahead of the first Democratic presidential debate on Tuesday. “Wow! That’s a reversal! I was against the Trans-Pacific Partnership months and months ago,” he said. “Secretary Clinton can justify her own reversal of opinion on this, but I didn’t have one opinion eight months ago and switch that opinion on the eve of debates.”

The polling on trade is mixed among Democrats, but a pro-trade group started by former Obama aides passed around a recent Pew poll this week suggesting that 45% of Democrats would be more likely to vote for a candidate who supports expanding free trade. Other polls have shown Americans skeptical of trade in general.

By waiting to oppose Obama’s trade authority, Clinton effectively lowered the stakes. The TPP still needs to be approved by Congress, but it likely already has enough votes to pass since Capitol Hill already signed off on a related Trade Promotion Authority bill, which required even more votes to pass.

As Sanders suggested, Clinton’s opposition would likely have been more damaging had it come before Congress voted on TPA earlier this summer, when it could have swayed on-the-fence Democratic lawmakers.

“Her opposition won’t move anyone who is currently in favor,” said Jim Kessler, the Senior Vice President for Policy at the pro-trade centrist Democratic think tank Third Way, which blasted Clinton’s opposition to the TPP. “Her support would help but her opposition maintains the status quo.”

Still, the AFL-CIO President Richard Trumka praised Clinton’s opposition and said it will boost the labor federation’s fight against TPP. “I applaud her for taking this step and choosing to embrace workers’ values. Her decision is a critical turning point, and will be invaluable in our effort to defeat TPP,” he said in a statement.

Clinton has been dueling with Sanders for endorsements from labor unions, with the AFL-CIO being the largest prize.

—NBC News’ Alex Jaffe contributed to this article.