The first Democratic primary debate offers Bernie Sanders an opportunity to elevate a subject that's thus far been largely absent from the debate: foreign policy.

This is not a traditional area of emphasis for Sanders, but it's one that he would be wise to try to exploit, since it is an area of emphasis for Clinton. It's also an issue where she's out of step with her party's base.

Barack Obama's 2008 primary victory over Clinton was multifaceted, but the difference between the two on the 2003 invasion of Iraq was critical and became the focal point for a broader argument about national security that proved to be a winning hand for Obama. In office, Obama has governed somewhat to the right of where he campaigned, while Clinton remains somewhat to the right of Obama.

That leaves a clear opening to try to revive Obama's original strategy of running to Clinton's left on national security. And it's one he needs. Sanders has overperformed expectations in Iowa and New Hampshire, but he's still way behind the frontrunner nationally. And with Clinton adopting populist stances on a range of economic issues, he could use a wider set of reasons for the Democratic base to look beyond the obvious choice. Foreign policy offers that. It's also an arena in which Sanders's record is arguably closer to that of a typical elected Democrat than is Clinton's — giving him an opening to remind party actors that there are things about Clinton they're not entirely comfortable with, and aspects of the Sanders agenda that they'd find reassuring.

Primaries matter most on foreign policy

A focus on national security in the debate poses two problems for Sanders:

He is not closely identified with the issue of foreign policy. Foreign policy is not currently a high priority for most Democrats.

And yet in many ways these are exactly the reasons a foreign policy focus in the debate would be smart for Sanders. Given the reality of congressional politics and partisan polarization, intraparty disagreements between Democrats don't have a ton of practical significance on many issues. Sanders favors a single-payer health-care system and a $15-an-hour minimum wage, but House Republicans aren't going to let either of those things happen. On most subjects, either candidate (or, indeed, basically any Democrat) would be pushing Congress in the same direction.

Foreign policy is different. Congress does less to constrain presidential action in the national security realm than in the domestic one. And in particular, there isn't an enormous amount a hawkish Congress can do to force a dovish president to use force more vigorously. Informal institutional constraints are real — the military brass and the intelligence community have ways of making their views known and advancing their interests —but this is still the area where the choice of which Democrat matters most in terms of policy outcomes.

Hillary Clinton is more hawkish than most Democrats

That foreign policy deserves to be a big deal for primary voters is a big deal for Hillary Clinton, because most signs are that her opinions on this subject are at odds with most Democrats.

Throughout Obama's first term she served as secretary of state and, according to most accounts, was on the hawkish wing of his administration. This generally involved taking stances that are unpopular with rank-and-file Democrats:

In mid-2009, then–Secretary of State Clinton was one of the key forces in the Obama administration advocating for a "surge" of new troops to Afghanistan. At the time, Gallup found that 62 percent of Democrats opposed sending more troops to the country.

In March 2011, she argued strongly for intervening to stop Muammar Qaddafi's slaughter of rebels in Libya. At the time, 57 percent of Democrats told Pew the US had no responsibility to stop the killing in Libya.

In 2012, Clinton and Gen. David Petraeus presented Obama with a plan for arming the Syrian rebels fighting Bashar al-Assad's regime. Only a tiny minority of Americans — 11 percent — supported the idea, according to a June 2013 NBC/Wall Street Journal. The poll didn't disclose an exact partisan breakdown, but Democrats and Republicans broadly agreed: "Whether you voted for Romney or Obama, they have the same opinion on Syria," Bill McInturff, one of the pollsters who conducted the poll, said.

More broadly, most Democrats say the United States spends "too much" on the military, a stance that would be out of step with Clinton's general view that the Obama administration has been too dovish.

And, of course, there was the invasion of Iraq — an issue that has fallen out of the headlines but that fueled insurgent campaigns by Howard Dean and Barack Obama in 2004 and 2008. Harping in 2015 on a vote that's more than a decade old would be petty and unwise. But Clinton's vote wasn't a one-off. Both before and after Iraq, she has taken a rosier view of unilateral American military force than the average Democrat.

Sanders's foreign policy views are banal

On domestic issues, Clinton is the very model of a modern mainstream Democrat, while Sanders has for decades positioned himself on the party's left flank. That's his brand, and he is neither capable of nor interested in changing it.

But on foreign policy, Sanders actually isn't especially left-wing.

As Zack Beauchamp has written for Vox, he's often angered some of his more left-wing admirers with his boring conventional liberal Democrat opinions. Sanders is critical of Benjamin Netanyahu, for example, and various aspects of Israeli policy but, like most Democrats, considers himself broadly pro-Israel. He backed Obama's anti-ISIS airstrikes and, though a bit leery of anti-terrorist drone strikes, says he sees the case for using them "in a very selective way."

On exactly the issue where Clinton is most to the right of the party, in other words, Sanders is squarely in the center and can plausibly argue that he's the true heir to the post-Iraq Democratic Party foreign policy that Obama inaugurated.

Syria could be especially fruitful ground for this. Clinton has doubly broken with Obama on that country's civil war, agreeing with Republicans that Obama's reluctance to arm Syrian rebels years ago was a mistake and joining Republicans in a call to establish a "no-fly zone" over Syria. Clinton's break with him on this point prompted him to quip back, "There is a difference between running for president and being president," suggesting that if Sanders were to offer the vigorous defense of his administration's policies that Clinton will not, he could move some important party actors to his side.

Pinning Clinton down

To Clinton's liberal critics (and to Clinton haters in the media), her stances on domestic issues have proven to be infuriatingly flexible. At earlier points in her career she was tough on crime, an ally of Wall Street, a proponent of free trade deals, and a marriage equality skeptic. But as the party consensus has shifted, she's shifted right along with it. To some, this is a lack of principle. But she's not alone in making these switches, nor are they necessarily unpopular. Her evolving views have evolved alongside those of her supporters and constituents and for essentially the same reasons.

But while she has disavowed her Iraq vote, she's only intensified her commitment to a generally hawkish outlook. Emphasizing these issues during a live debate would take Sanders out of his comfort zone, but it would also force Clinton to address topics where she hasn't tailored her message to the Democratic primary electorate and where her convictions and instincts are simply out of step with the party base.