Former Maryland governor Martin O'Malley probably can't win the Democratic presidential nomination in 2016. But he can make things very uncomfortable for Hillary Clinton. Of late, he seems very committed to doing just that.

The latest gambit by O'Malley came Wednesday when he sent an e-mail to supporters pledging his opposition to the Trans-Pacific Partnership. "To me, opposing bad trade deals like TPP is just common sense," O'Malley wrote. "American workers whose jobs could be on the line right now are owed more than lip service. They deserve to know where leaders stand."

Whatever "leaders" could he be talking about?

Maybe Clinton, who in New Hampshire this week has avoided taking an outright position on TPP. “Any trade deal has to produce jobs and raise wages and increase prosperity and protect our security," Clinton said in a statement that is simultaneously true and meaningless.

O'Malley crafted an unapologetically liberal record during his time as Maryland's governor, and without Massachusetts Sen. Elizabeth Warren in the race, he seems likely to be Clinton's liberal conscience in the contest. While Clinton's team is doing everything in its power to cast her as a longtime populist fighting Wall Street on behalf of the little guy, that's going to be a very tough sell.

Take trade, for example. As Republicans gleefully point out, Clinton called the TPP the "gold standard" of trade agreements during her time at the State Department. And Hillary Clinton is still dealing with some level of distrust about labor organizations due to her husband's 1994 signing of the North American Free Trade Agreement. Then-Sen. Obama attacked Clinton for her support of NAFTA during the 2008 Ohio primary, and fact-checkers dinged her for not acknowledging that she had shifted her position on the trade agreement. (It's worth remembering that John Edwards was the organized labor candidate of that 2008 race.)

O'Malley hitting Clinton as not liberal enough -- whether it's on trade or anything else  -- isn't great but it also won't cause her to lose the nomination. What could be a more fruitful (if you are O'Malley) or damaging (if you are Clinton) approach is to cast her unwillingness to take a firm stand on an issue like TPP as indicative of a politician who lacks any core beliefs and simply does what is politically expedient at all times.

O'Malley's rhetoric -- and that of a top aide of his who said there was "no hedging" in his position on TPP -- seems to be headed in that direction these days. And we know from the 2008 campaign just how problematic being painted as overly political in all things can be for Clinton.  Remember this?

That moment -- at the end of an otherwise stellar debate for Clinton -- was the beginning of her long slide and defeat. Why? Because it crystallized for many people exactly what they didn't like or trust about her. She looked too political by (at least) half.

For Clinton then, the true danger of O'Malley's increasing outspokenness on issues is not that he will paint her as not liberal enough. Most polling suggests that the bulk of liberals like her and will vote for her. Instead, an emboldened O'Malley could remind voters of the calculating political side of Clinton that many of them do not like. It's O'Malley's boldness that's the real threat to Clinton, not his liberal positions.