Asked in an interview this week about her presidential ambitions, Hillary Clinton gave an answer that qualified as a howler even by Clinton standards: "I'm not thinking about it."

Clinton is widely considered the presumptive Democratic nominee for president in 2016. Given the atavistic chaos that afflicts the Republicans, many view her as the virtual president-elect. Time magazine ran a cover story this month headlined "Can Anyone Stop Hillary?" The New York Times Magazine followed with a cover story of its own, the latest in a stream of media coverage of the juggernaut that is Clinton's unannounced presidential campaign.

One of the surest signs that Clinton is running for the presidency is her refusal to take a position on the greatest geopolitical question now facing the United States. President Obama and Secretary of State John Kerry are engaged in a high-stakes effort to end 35 years of hostility between the United States and Iran. Debate about this initiative is intense in Washington. No one, however, knows the opinion of the woman who was Kerry's immediate predecessor and is evidently seeking to govern the United States beginning in 2017.

Kerry has asserted that negotiations with Iran are "one of those hinge points in history," and argued that they give the United States "a chance to address peacefully one of the most pressing national security concerns that the world faces." Senator Dianne Feinstein, who heads the Senate Committee on Intelligence, has warned that those who seek to block reconciliation are on a "march toward war."

Sentiments are just as strong on the other side. Prime Minister Benjamin Netanyahu of Israel has denounced negotiation with Iran as a "historic mistake" that is making the world "a more dangerous place". His partners in Washington vigorously echo that view. One of them, Senator Mark Kirk, has accused Obama of behaving "like Neville Chamberlain" and charged that he is setting the stage for "a large and bloody conflict in the Middle East involving Iranian nuclear weapons".

This is the most far-reaching foreign policy debate that has broken out in Washington in more than a generation. The stakes for the United States, Iran, the Middle East and the world are huge. American politicians are falling over one another to press their views. Clinton is the glaring exception.

Throughout her career, Clinton has stayed well within the Washington paradigm on foreign policy issues. Like many American politicians who came of age during the Cold War, she takes an us-versus-them view of the world. She has never dissented from the Washington chorus that portrays Iran as an irredeemable font of evil. Had she remained on the job as secretary of state rather than resigning and paving the way for Kerry, the United States would certainly not have made an effort to engage Iran.

Now that a preliminary agreement has been struck and international inspectors are monitoring Iran's retreat from its nuclear program, it is reasonable for Americans to expect their leaders to say whether they favor or oppose this process. That is especially true of Clinton, who until a year ago was the global face of US foreign policy. Yet her silence has been deafening.

Clinton has a habit of not taking any position until it is clear which position will be most politically beneficial. "No doubt we will find out HRC's true convictions just as soon as her focus groups report in or her major donors tell her what to think," Stephen Walt wrote in his Foreign Policy blog.

Here lies the dilemma. A strong statement by Clinton in favor of reconciliation would be a game-changer in Washington. She would be giving a centrist, establishment endorsement of her former boss's most important foreign policy initiative. That would provide political cover for moderate Democrats terrified of antagonizing the Netanyahu government and the American Israel Public Affairs Committee, which is leading the anti-reconciliation campaign in Washington.

Such a statement, however, would risk outraging pro-Netanyahu groups and individuals who have been among Clinton's key supporters since her days as a Senator from New York. Having spent years painstakingly laying the ground for a presidential campaign, she does not want to risk a misstep that would alienate major campaign contributors.

Clinton's choice is clear. If she opposes détente with Iran, she will look like a warmonger who prefers confrontation to diplomacy. If she supports it, she will alienate a vital part of the base she is relying on to finance her presidential campaign. With this in mind, she has chosen to remain silent on the central foreign policy issue of the age. It is a classic act of political cowardice – the kind that often leads to victory at the polls.