WASHINGTON -- A specter is haunting the presidential race -- and it is not just the economy. It is the specter of a nuclear Iran.

Economic downturns are wrenching, but eventually cyclical. Nuclear proliferation is more difficult to reverse, creating the permanent prospect of massive miscalculation and tragedy. America's next leader may be known to history as the president who had to deal with Iran.

This topic received glancing attention in the second presidential debate. Barack Obama called a nuclear Iran "unacceptable." John McCain said this would raise the prospect of "a second Holocaust." But neither man seriously confronted the choices ahead.

Days earlier, at an event at the Nixon Center here, the former chief weapons inspector for the United Nations, David Kay, delivered a bleak assessment of Iranian capabilities and intentions. The Iranian regime, he argues, is about 80 percent of the way toward its nuclear goals -- perhaps two to four years from "effective, deployable weapons."

Kay believes that the reaction to this threat by both political parties is unrealistic. By simply saying a nuclear Iran is unacceptable, America is set up for a choice between "suicide" (a disastrous military attack on Iran) and "humiliation" (a galling acceptance of the unacceptable). Instead, Kay calls for a new round of "skillful diplomacy" to convince Iran to stop at what he calls "virtual capability" -- a global recognition that it could produce nuclear weapons in short order, without all the drawbacks caused by actually producing those weapons.

But this would be the third major attempt at diplomacy, not the first. Russia has offered Iran enriched nuclear material for use in its civilian nuclear plants in exchange for abandoning its fuel enrichment program. Iran refused, demonstrating, at the least, that it wants the technical know-how -- the "breakout capability" -- to produce nuclear weapons. The Bush administration has offered direct, face-to-face talks with Iran if it would merely suspend (not abandon) its enrichment program. This also has been turned down. Another diplomatic effort -- perhaps offering normalized relations and the lifting of sanctions in exchange for Iran's full cooperation -- might further isolate Iran if it refuses the deal. But even many supporters of such an initiative admit Iran is likely to refuse.

So Kay seems resigned to a policy of containment -- holding Iran directly responsible if it transfers nuclear weapons to terrorists, providing nuclear guarantees to our friends in the region so they don't feel pressured to develop their own. Past nuclear proliferation to nations such as France and India, he argues, proved less destabilizing than many first feared.

The problem with this approach? Iran may be a different proliferation threat from any we have faced before. The regime cultivates ties to violent non-state proxies in Lebanon, Iraq, Afghanistan and the Palestinian territories. While in some ways calculating, its leaders also seem drawn toward dangerous terrorist adventures -- such as blowing up U.S. troops in Beirut or Jewish community centers in South America. Iran's religious radicalism introduces an unpredictable element of irrationality. And some future conflict between a nuclear Iran and a nuclear Israel could easily and quickly escalate.

What are the alternatives? Attempting to destabilize the Iranian regime from within -- by covert action and support for dissidents -- does not seem realistic on a four- or five-year timeline. American capabilities in this regard are limited, and Iranian repression of reformers is ruthless.

So if a nuclear Iran is truly unacceptable, we may be left with the use of military force. And this only seems credible under narrow circumstances. As Gary Samore, my colleague at the Council on Foreign Relations, points out, Iran can move from breakout capability to the development of nuclear weapons in only two ways. It can do the final enrichment of weapons-grade material at some secretly constructed facility with a few thousand hidden centrifuges -- a difficult and risky proposition. Or it can quickly convert its known centrifuges for such production. This would likely take a few weeks, and require the expulsion of international inspectors. During this short time lag, Iran's intentions would be fully revealed, and the case for bombing its facilities would be strongest.

This may be the true test of the next president -- a few days to make one of the most consequential decisions in modern history. It is difficult to imagine why anyone would covet the responsibility for that choice -- but it is necessary to discern who is best prepared to make it.