Washington ? Iran’s ayatollahs have issued a stream of defiant messages stressing their determination to join “the nuclear club,” no matter what price they may pay in international isolation. Their warnings push a camouflaged crisis into public view and demand a coordinated global response.

Iran’s now unmistakable nuclear ambitions will also soon demand the attention and responses of George W. Bush and John Kerry. Iran, not Iraq, may well be the foreign policy test of leadership as the U.S. presidential campaign brings the candidates into direct debate.

The verbal thunder from Tehran seems to signal an end to diplomatic efforts by the European Union, supported by Washington, to negotiate the Iranians out of developing a nuclear weapon. While maintaining outward confidence that their talks were making headway, EU diplomats had concluded privately in recent weeks that their efforts had run aground.

“The Iranians are determined to continue until they know they can assemble a bomb within hours should they need it,” says one European official aware of the negotiations. “No diplomacy will stop that. We may in fact have to aim for that outcome as a best-case scenario, by bargaining them into stopping just short of final assembly and testing.”

Agreeing to live with an Iran that is a screwdriver’s turn away from the bomb would be a bitter pill to swallow. It would accommodate a charter member of President Bush’s “axis of evil” and a sworn mortal enemy of Israel. And it would undermine the goals and terms of global non-proliferation agreements that aim at halting the spread of nuclear weapons technologies to nations that do not now possess them.

But it is no longer an unthinkable notion. Nothing else — not Bush’s harsh rhetoric, Europe’s diplomacy, or traditional non-proliferation agreements — has stopped Iran’s secret work on nuclear weapons in recent months, United Nations inspections and Western intelligence reports have determined.

In a letter published in the Tehran press on Tuesday, Iranian President Mohammad Khatami told the leaders of France, Germany and Britain that they must now stop pressuring his country to drop its nuclear program, which Khatami claims is only for producing energy. His letter contained an implicit threat to withdraw from the Non-Proliferation Treaty if the Europeans did not comply.

This followed a pugnacious declaration by Foreign Minister Kamal Kharrazi demanding that Iran “has to be recognized by the international community as a member of the nuclear club. This is an irreversible path.”

The combined efforts by the EU negotiators and by U. N. inspectors did accomplish this much: It slowed the Iranian effort by about a year. Western and Israeli estimates, which originally foresaw 2005 as the earliest probable completion date for the assembly of a nuclear weapon, have been revised to 2006.

With no obvious, workable military option, the next American president will be confronted with a growing feeling among Europeans that the best available strategy may be trying to keep an Iranian finger off the trigger of fully developed and deployed nuclear weapons.

Iran would then come to resemble Pakistan, which froze its development at the screwdriver stage in 1989 and then exploded its first device nine years later to respond to India’s nuclear testing. The thrust of international diplomacy since the 1998 tests has been to prevent deployment of nuclear weapons by India or Pakistan.

Iran is a living, moving foreign policy quandary that is just over the horizon in voters’ concerns. Kerry and Bush should move now to spell out the actions that would prevent this crisis from worsening as the campaign proceeds. That forces each candidate to think forward and gives voters a chance to see each’s judgment at work in real time.

The Bush administration’s credibility on managing Iraq has fallen so far that all Kerry has to do to score points is utter that country’s name. His criticisms are seemingly validated daily by the disasters splashed on the front pages and television screens. By cautiously shadowing Bush’s policies and condemning their implementation, Kerry deftly avoids getting in the way of the beating that Iraq is giving the president at this point.

Kerry’s untested (and largely unrealistic) “solutions” of getting France and Germany more involved and providing better training to Iraqis look credible only by comparison to Bush’s recent floundering. Kerry can parry questions about what he would do in Iraq by dwelling on what Bush did and how it seems to be failing.

Iran’s evolving crisis demands fresh thinking and articulation in the framework of broader Persian Gulf and global non-proliferation policies. Mr. Candidates, your responses, please.

— Jim Hoagland is a columnist for Washington Post Writers Group.