Clinton's approach so far has been to stay vague. "She will be watching closely to see what is being done to crack down on currency manipulation, improve labor rights, protect the environment and health, promote transparency and open new opportunities for our small businesses to export overseas," her campaign said Friday. On Tuesday in New Hampshire, the candidate herself added, “Any trade deal has to produce jobs and raise wages and increase prosperity and protect our security. We have to do our part in making sure we have the capabilities and the skills to be competitive.”

Those don't sound like full-throated endorsements, as O'Malley implied. He followed up that tweet with an email to supporters with the subject line "Hard choice?" (Clinton's memoir, released last June, was Hard Choices.)

Then there was this awkward exchange in a press gaggle with White House spokesman Eric Schultz on Wednesday:

... Do you consider Hillary Clinton an ally on this trade stuff? Evan, I'm going to side with you on this. I believe that the labor, environmental and human rights concerns that many Democrats have voiced, the President takes to heart. And he would not sign a deal unless those protections are in place. If you look at the TPA agreement that was introduced in a bipartisan way in the Senate, we believe that’s the most progressive in history and that’s why the President is encouraged by it. So Secretary Clinton and President Obama are on the same page with trade? Well, look, I believe that if you look at the points that are being raised in terms of human rights, environmental protections, labor protections, that those are important priorities of this President. So I haven’t seen anything to suggest any distance.

Here's the thing: Clinton has largely adopted the central liberal critique. Warren, at least in theory, isn't opposed to all free-trade deals‚ but she warns that the TPP could be a bad deal and that the contents of the deal need to be public so that all Americans can read them and make up their minds. That's where O'Malley is too. (In a video in that email, O'Malley said, "I'm for trade, and I'm for good trade deals. But I'm against bad trade deals like the Trans-Pacific Partnership.") Now Clinton has joined them, sort of, in saying that the deal requires environmental and labor protections, while not quite calling for total transparency.

Will that half-a-loaf approach satisfy constituencies like the AFL-CIO, though? The group's president, Richard Trumka, blasted the deal at a hearing Tuesday. "The livelihoods of workers are at stake here," he said. "We need a different deal." Remaining vague has political upsides, and it avoids the political circus that would come with an outright break from the president. But it won't help her to convince progressives she's one of them. Unlike same-sex marriage, for example, free trade isn't usually a top-tier issue, but it's fairly easy for Clinton to stand behind marriage equality in 2015, when the issue is largely settled for many people. Taking a stance on the TPP is playing with live ammunition.

Clinton still enjoys overwhelming support in the polls, and the fight over the TPP seems unlikely to change that. But it highlights the difficulties she faces as she attempts to win over her party's progressive wing, without alienating other constituencies. It is proving a difficult deal for Clinton to negotiate, as she attempts to fast-track her bid for the Democratic nomination.