I don't really want to just quote an excerpt this long, but James Traub really nails the difference between Barack Obama and Hillary Clinton in terms of their supporters in the world of foreign policy:

The United States has had only one foreign policy and one national-security strategy since the transforming events of 9/11 — and this set of doctrines has been shaped by the very distinctive worldview of George W. Bush and Dick Cheney and the men and women around them. The great project of the foreign-policy world in the last few years has been to think through a “post-post-9/11 strategy,” in the words of the Princeton Project on National Security, a study that brought together many of the foreign-policy thinkers of both parties. Such a strategy, the experts concluded, must, like “a Swiss Army knife,” offer different tools for different situations, rather than only the sharp edge of a blade; must pay close attention to “how others may perceive us differently than we perceive ourselves, no matter how good our intentions”; must recognize that other nations may legitimately care more about their neighbors or their access to resources than about terrorism; and must be “grounded in hope, not fear.” A post-post-9/11 strategy must harness the forces of globalization while honestly addressing the growing “perception of unfairness” around the world; must actively promote, not just democracy, but “a world of liberty under law”; and must renew multilateral instruments like the United Nations.

In mainstream foreign-policy circles, Barack Obama is seen as the true bearer of this vision. “There are maybe 200 people on the Democratic side who think about foreign policy for a living,” as one such figure, himself unaffiliated with a campaign, estimates. “The vast majority have thrown in their lot with Obama.” Hillary Clinton’s inner circle consists of the senior-most figures from her husband’s second term in office — the former secretary of state Madeleine Albright, the former national security adviser Sandy Berger and the former United Nations ambassador Richard Holbrooke. But drill down into one of Washington’s foreign-policy hives, whether the Carnegie Endowment or the Brookings Institution or Georgetown University, and you’re bound to hit Obama supporters. Most of them served in the Clinton administration, too, and thus might be expected to support Hillary Clinton. But many of these younger and generally more liberal figures have decamped to Obama. And they are ardent. As Ivo Daalder, a former National Security Council official under President Clinton who now heads up a team advising Obama on nonproliferation issues, puts it, “There’s a feeling that this is a guy who’s going to help us transform the way America deals with the world.” Ex-Clintonites in Obama’s inner circle also include the president’s former lawyer, Greg Craig, and Richard Danzig, his Navy secretary.

The first of the Clinton people to notice this rising political star was Anthony Lake, national-security adviser in Bill Clinton’s first term. Lake says that he was introduced to Obama in 2002 when the latter had just begun considering a run for a Senate seat. Impressed, he began contributing ideas. When Obama came to Washington as a senator and joined the Foreign Relations Committee, Lake continued to work with him on occasion. Like others, Lake was impressed not so much by Obama’s policy prescriptions as by his temperament and intellectual habits. “He has,” Lake says, “the kind of mind that works its way through complexities by listening and giving some edge of legitimacy to various points of view before he comes down on his, and that point of view embraces complexity.” This awareness of complexity felt like a kind of politics itself and a repudiation of the Bush administration’s categorical thinking.

Obama spoke out against the impending war in Iraq in the fall of 2002; and those members of the Democratic establishment who, like Lake, also opposed the war came to view him as a kindred spirit. Susan Rice, a former assistant secretary of state in the Clinton administration who, along with Lake, heads up Obama’s foreign-policy team, says, “You were considered naïve, wrong, weak, stupid to oppose that war.” Hillary Clinton (and John Edwards) voted for the war. Obama’s opposition to it showed Rice “a willingness not to be bound by conventional wisdom and the well-trod path.”

This is all quite right. And it's important to recall that this hawk/dove split and the elite/rank-and-file split have some causal interaction. Back in 2002, the Democratic establishment found itself trapped in this vicious cycle. Most rank-and-file members of congress were ready to oppose the war. But the leadership in the House and the Senate was backing it. And the campaign committees were advising challengers and vulnerable members to back it. And the conventional wisdom said that anyone who wanted to be elected president had to back it. And so were most of the media celebrities focusing on foreign policy — Holbrooke and Albright and Pollack and O'Hanlon. In part, political leaders backed the war because these "experts" were backing it, and in part the celebrity experts were backing it because the politicians they were courting were backing it.

But it all blew up in everyone's face. The war was, substantively, a disaster. And it became hard to take advantage of the disaster because so many leading Democrats had backed it.

And in foreign policy terms, though Clinton certainly counts some war opponents and some younger rank-and-file people, she and her campaign fundamentally represent continuity with that seem set of political and policy elites who were running the show in 2002 and 2003. Obama represents a break from that; a turn toward people who think a different way, who probably aren't as famous but just might know what they're talking about, and perhaps even more important than that to people whose thinking isn't hobbled by an unwillingness to break with past positions.