(CNN)Well, the thought experiments are finally over. Hillary Clinton's online declaration for president means the focus will now shift to the campaign and to what kind of president she might be. And nowhere will the speculation be greater than in the area of foreign policy, certain to be a major issue in the upcoming campaign. Should she win, Clinton will add to her potential "firsts": first woman president; the first president who had been a first lady. There's another one, too: the first secretary of state to become president since James Buchanan. Only a handful of the nation's top diplomats have gone to the White House (Thomas Jefferson; James Madison; James Monroe; John Quincy Adams; Martin Van Buren; and Buchanan). And none in the 20th century, a curious fact that suggests politics and statecraft are quite different animals. Too few of our secretaries of state have had the necessary experience of elected office or the desire, ambition and temperament to compete for the top job. Still, on balance, Clinton's tenure at the State Department should be a real advantage during the campaign, particularly when compared with the absence of foreign policy experience among her prospective Republican rivals. But this will be no cake walk for her on the foreign policy side. Clinton will have to negotiate and traverse several tricky and rocky paths to ensure that her State Department career remains an advantage and doesn't turn into a liability. Here's why: . The last president to serve in a top job in the administration of his immediate predecessor was George H.W. Bush.  Circumstances were different then. Far from wanting to distance himself from Reagan, Bush 41 saw merit in reinforcing the association with a president who dominated his day in a way few chief executives have. Clinton will face a harder balancing act: how to stand by the policies that she helped craft in Obama's first term and still separate herself from an administration her opponents will blast as weak, vacillating and fairly or not, responsible for a world that is seen to be much worse, especially in the Middle East and with Russia, than when President Obama took office. In the process, she'll have to define her own approach to the world. Indeed, given the President's vulnerabilities on foreign policy, she can't afford to be seen as Obama's third term. In her memoir "Hard Choices," she's already laid the foundation for distancing herself from Obama on issues such as doing more for the Syrian opposition and being tougher on Russia's Vladimir Putin. And she can legitimately work to sharpen those differences without seeming to walk away from policies she supported and leaving herself open to charges that she has no principles, only politically expedient tactics. Too much loyalty to a president who is unpopular among independents will hurt her. But so will flip-flopping. Nowhere will her challenges be greater than on Iran. Clinton presided over the secret channel that laid the basis for the November 2013 interim accord and led directly to the putative understandings reached between the United States and Iran earlier this month in Lausanne, Switzerland. And she certainly can take credit for pushing tough sanctions that forced Iran to the table. The problem, of course, is that her Republican opponents and more than a few Democrats hate what the negotiations have produced. Much less enamored with the Obama's "let's engage our enemies" trope, Clinton may have doubts herself. The good news is that there won't be an agreement for months. So for now, she can back the importance of tough negotiating and even tougher sanctions or worse, should the Iranians cheat.  But sooner or later, perhaps as early as June if there's an agreement, Clinton will have to take a stand on what may well be a very problematic and unpopular accord and what Congress' role should be. Her opponents, the Israelis, and much of the organized Jewish community, will portray it as even worse, and that is likely to be somewhat problematic and will be hyped as even worse. And this will put her at odds with traditional friends and supporters in the Jewish community. Frankly given her political interests, it would be easier for her candidacy if the agreement fell apart and she could campaign on a tough anti-Iranian message, hammering the mullahs' repressive policies at home and their mischief-making in the region. She may not be that lucky. Still she will have an advantage in dealing with the pro-Israeli community. Unlike President Obama, the Clintons have strong credentials on the Israeli issue. And that will help somewhat in trying to walk a narrow line between a negotiating process with Iran that Clinton launched and its fruits, which are seen by many Israelis and American Jews right now as too generous to the mullahs. Clinton's time at the State Department should help her in a presidential campaign where Americans are looking for strong and prudent leadership in foreign policy and more adult supervision in the White House. The Republicans will try and show that Benghazi and the email controversy have tarnished her image as secretary and claim she didn't accomplish much. Clinton wasn't a Henry Kissinger or a James Baker. But given the problems she confronted -- Iran, stalled Israeli-Palestinian two-state negotiations (none ready for any kind of solution or breakthrough) and a president who dominated rather than delegated foreign policy, she performed ably enough. She improved the nation's image and pushed 21st century issues such as women's rights, youth, and the environment. Her critics will dismiss all this as a kind of naive planetary humanism. Still, four years as secretary of state will help her project the kind of confidence and competence that will appear to many as an important credential  to lead America in a dangerous and turbulent world.  And in a presidential campaign where none of her opponents has her long experience in international affairs, that can only help. Clinton's biggest challenge on the campaign trail and in office should she win is whether she can develop a foreign policy vision and an effective approach to the world that strikes a better balance between the risk-readiness of George W. Bush and the risk-aversion of Barack Obama.  And given the cruel and unforgiving nature of the world America now inhabits, this will be no easy task.
Miller: The former secretary of state has to decide whether she's going to differ with Barack Obama's handling of foreign policy . He says her experience overseas could be an asset as long as she doesn't get ensnared by controversy over Obama policies .