Hillary Rodham Clinton has joined her Democratic presidential rivals in opposing President Barack Obama's Pacific trade deal, delivering a major blow to a Democratic president as she works to court her party's liberal base.

The now-united opposition from the Democratic presidential field leaves Obama in the uncomfortable position of watching a Democratic presidential debate next week in which none of the major candidates is willing to defend a deal that the White House sees as a key piece of his presidential legacy. The Trans-Pacific Partnership trade accord has enraged the labor unions, environmentalists and other liberal constituencies whose support is crucial in the Democratic primary contest.

Yet Clinton's position marks a sharp reversal from a deal she backed as the Obama administration's top diplomat as she works to appeal to skeptical liberals.

"I appreciate the hard work that President Obama and his team put into this process and recognize the strides they made," Clinton said in a statement. "But the bar here is very high and, based on what I have seen, I don't believe this agreement has met it."

Clinton promoted the deal in dozens of appearances as secretary of state during Obama's first term in office -- a turnaround that was not lost on her primary opponents.

"Secretary Clinton can justify her own reversal of opinion on this but I can tell you that I didn't have one opinion eight months ago and then switch that opinion on the eve of debates," said Clinton's presidential rival, former Maryland Gov. Martin O'Malley.

Clinton's support for trade deals has seemed to fluctuate with the political calendar.

As first lady, she trumpeted the North American deal brokered by her husband, telling unionized garment workers in 1996 that the agreement was "proving its worth."

Her support for trade pacts began softening during her time as a New York senator, when she voted for trade agreements with Chile, Singapore, Oman, and Morocco but opposed the Central American Free Trade Agreement.

In a November 2007 presidential debate, Clinton described the North American agreement, with Canada and Mexico, as "a mistake" and called for a "trade timeout."

In that vein, she said she opposed then-pending trade agreements with Korea, Columbia, and Panama. But fast-forward to July 2011 when, as secretary of state, she described those three deals as "critical to our economic recovery."

She also repeatedly lent her support to the Pacific trade initiative being pushed by Obama, at that time, describing the deal during a 2012 trip to Australia as the "gold standard in trade agreements."

Clinton aides know she must tread lightly when it comes to criticizing Obama, given that much of her strategy relies on the still-loyal coalition of African-Americans, Latinos, women and younger voters that twice elected Obama. But at the same time, they say she must find ways to distinguish herself -- and undercut Republican attacks that Clinton would simply provide a third Obama term.

But the White House doesn't deny that Clinton's new distance has sometimes created awkwardness for the president. The Clinton campaign gave White House aides a heads up Wednesday before she made the comments, according to a White House official who would not be named discussing the private conversation.