While the handling of the September terrorist attack in Benghazi, Libya, continues to capture the attention of the campaigns and the media, the most important foreign policy issue of the election has remained largely unaddressed.

BROOKS KRAFT / CORBIS FOR TIME President Barack Obama and Republican presidential candidate Mitt Romney participate in the second presidential debate at the David Mack Center at Hofstra University in Hempstead, New York, Oct. 16, 2012.

While the handling of the September terrorist attack in Benghazi, Libya, continues to hold the attention of the campaigns and the media, the most important foreign policy issue of the election has remained largely unaddressed: When and how would the U.S. go to war with Iran to prevent it from getting a nuclear weapon?

By most accounts, if Iran continues on its current path, it will be capable of producing a nuclear weapon in the term of the next President. Publicly, Barack Obama and Mitt Romney have committed to preventing Iran from getting a weapon and have said military action remains a possibility if necessary. Privately, senior advisers to both men admit that any military action would have catastrophic affects on the international economy, stability in the region and U.S. interests around the world.

Determining the moment when the dangers of a potential Iranian bomb outweigh the dangers of military intervention may be the most important judgment the next President will make.

Tonight the final presidential debate will bring the issue into focus. Obama Administration officials tell the New York Times that Iran has agreed to engage in direct negotiations over its nuclear program. That revelation will frame the issue in a way that helps Obama: the Administration says it has not agreed to talks but can claim a long-sought concession from Iran. Romney will have to argue that the Administration cannot be trusted to be tough with Iran in talks.

(Photos: Political Photos of the Week, Oct. 12-18)

One way to avoid a canned exchange on this issue would be to ask both men to assess the efficacy of the current sanctions regime. In a clear, early diplomatic victory, the Obama Administration succeeded in implementing global sanctions against Iran that went well beyond what anyone thought possible at the end of the presidency of George W. Bush. The U.N. and E.U. agreed to and implemented various oil, trade and finance sanctions; the Obama Administration persuaded some individual countries to go further than the new regimes required; and it got others, especially rising powers China and India, to take their own steps to isolate Iran.

But the sanctions regime failed to change Iran’s behavior. Over the past four years, the Iranian nuclear program hasn’t slowed down; it has accelerated. The mullahs cracked down on political dissent during the so-called Green Revolution, and international talks stalled. Sabotage and assassination seemed if anything to harden Tehran’s determination to pursue nuclear-weapons capability. As they had before in Iraq and elsewhere, sanctions seemed to be failing as a policy.

But this month Iran was hit with a currency crisis that cast the sanctions effort in a new light. Amid rampant inflation and a dramatic devaluation of the Iranian currency, protests erupted and resistance to the regime built among the business class. A key question now is whether the crisis is a momentary setback for Iran or a sign that sanctions are finally working. If Tehran’s concession on talks is the result of sanctions, a deal may be possible. If not, the Iranians are likely stalling while they pursue their nuclear program.

Required reading for both campaigns and Monday’s debate moderator, Bob Schieffer, should be the latest nonpartisan Congressional Research Service report (pdf) by veteran analyst Ken Katzman on the state of Iran sanctions (via Steve Aftergood’s indispensable “Secrecy” blog, here).

Katzman concludes, “Many judge that Iran might soon decide it needs a nuclear compromise to produce an easing of sanctions because:

Oil exports provide about 70% of Iran’s government revenues and Iran’s oil exports have declined sharply as a result of the sanctions. A European Union embargo on purchases of Iranian crude oil that took full effect on July 1, 2012. Previously, EU countries were buying about 20% of Iran’s oil exports. This embargo is coupled with decisions by several other Iranian oil customers to substantially reduce purchases of Iranian oil in order to comply with a provision of the FY2012 National Defense Authorization Act (P.L. 112-81).

Together, these sanctions have reduced Iranian oil exports to about 1 million barrels per day as of October 2012, a dramatic decline from the 2.5 million barrels per day Iran exported during 2011. This loss of sales has caused Iran to reduce oil production, to the point where it is producing less oil than is Iraq.

The loss of hard currency revenues from oil — coupled with the cut off of Iran from the international banking system and the reported depletion of Iran’s foreign exchange reserves — caused a collapse in the value of Iran’s currency, the rial, in early October. That collapse prompted street demonstrations and a halt to commerce by merchants who are uncertain how to price their goods. In response, Iran has tried to impose currency controls and arrested some illegal currency traders, although these steps are unlikely to restore public confidence in the regime’s economic management. Other oil producers, particularly Saudi Arabia, are selling additional oil to countries cutting Iranian oil buys, thus far preventing the lost Iranian sales from raising world oil prices.

As the U.S. limps home from wars in Iraq and Afghanistan, Romney and Obama should address explicitly whether they think sanctions are working, whether they should be expanded and under what circumstances they would abandon them in favor of military action.