As I noted yesterday, Hillary Clinton’s decision to criticize President Obama’s foreign-policy failures indicates that she believes the field has been cleared for her to take the Democratic presidential nomination in 2016. But while there’s little doubt she will be her party’s choice, her shift to the center this early in the process guarantees that someone from the left will step up to challenge her. And though she may have scared all the credible contenders, it would be unwise for her camp to dismiss the impact of a Bernie Sanders candidacy on her chances to win the presidency.

If Clinton isn’t shaking in her boots about the prospect of facing off against the Vermont senator, it’s hard to blame her. Even the self-proclaimed socialist with the thick New York accent doesn’t think he is likely to be elected president. In many ways, Sanders is a political consultant’s nightmare of a candidate with extreme views and about as much charisma as a soggy potato. But it is precisely because he has no plans for future runs at high office that he is potentially so dangerous to Clinton’s ultimate goal of winning the general election.

Sanders had already been making noises about his willingness to run in 2016 in order to ensure that the left’s issues are heard. But in an interview published on Yahoo News yesterday he made clear that Clinton’s attempt to leave her party’s base out in the cold would draw a stiff rebuke from liberals and ensure that the primary process will not be a coronation.

While Sanders did not go so far as to promise to try to stop Clinton or to change the direction of her candidacy, his criticism was pointed enough to encourage speculation that he is prepared to run. According to Sanders, Clinton is not sufficiently interested in the issue of income inequality:

“What is her agenda? I don’t know. You don’t know. She hasn’t said.”

Short of a tilt back to the left, it appears Sanders is willing to be the one to carry the water for the liberal base in 2016. While no one should think he is likely to beat out Clinton, a Sanders candidacy does have the potential to play havoc with Clinton’s strategy.

First, it should be understood that the very fact that Sanders isn’t serious about the presidency would make him dangerous. Unlike some other potential Democratic contenders, Sanders isn’t worried about offending the Clintons or anyone else in the party. Moreover, as a figure of the left and avowed independent, Sanders is more interested in ideology than Democratic Party unity or winning general elections.

That will mean he won’t shy away from hot button issues, both foreign and domestic, in an effort to either hammer the more centrist frontrunner for her apostasy or to embarrass her into tacking back to the left.

What Clinton and perhaps other Democrats who would prefer to avoid messy primaries may forget is that despite the discipline exhibited by so many liberals in recent years, there’s nothing like a hopeless underdog presidential challenge to engage the sympathies of primary voters. That means Sanders doesn’t have to appear presidential or even to make all that much sense in order to gain the sympathy of many Democrats. And, unlike conservative outliers running for the Republican nomination in recent years, Sanders won’t be pilloried as a nut case the way someone like Michele Bachmann was when she ran in 2012.

Instead, Sanders will be portrayed as a feisty truth-telling codger who won’t lie down for the Clinton machine. Though he has no chance of ever being nominated, he could roll up impressive totals in early voting states like Iowa and New Hampshire where minority voters—more inclined to support Clinton rather than an ideologue like Sanders—won’t play as much of a role.

That means Clinton’s strategy of attempting to run a general election-style campaign for president from start to finish may not play out the way she thinks. Indeed, her neoconservative tilt on foreign policy may only egg on Sanders and gain him votes from many Democrats who will feel that casting a symbolic ballot against Hillary will prevent her from straying too far from left-wing orthodoxy.

Rather than providing a stress-free primary season as Clinton thought, a Sanders challenge may actually be far more trouble for the former first lady than one from a more mainstream Democrat like former Maryland Governor Martin O’Malley, who is more afraid of offending the Clintons and has an eye on the future.

As we saw in 2008, Clinton is not merely vulnerable to an attack from her left flank; she is also a poor candidate who finds it hard to avoid trouble on the stump or even with friendly interviewers. Though Sanders may not be able to stop Hillary, he could make her life miserable. The path back to the White House for Clinton seems smooth, but a spirited left-wing challenge could undo all her plans and force her to not only spend time and resources battling other Democrats but undermine the united party front that appears ready to back her effort to become the first female president.