President Obama should take a page from Ronald Reagan’s playbook in winning the final inning of the Cold War. Obama can challenge President Mahmoud Ahmadinejad to put his enriched uranium where his mouth is — by stopping all Iranian enrichment of uranium beyond the 5 percent level.

A quarter-century ago, Soviet leader Mikhail Gorbachev was touting a new “glasnost”: openness. President Reagan went to Berlin and called on Gorbachev to “tear down this wall.” Two years later, the Berlin Wall came tumbling down and, shortly thereafter, the Soviet “evil empire” fell as well.

While in New York for the opening of the U.N. General Assembly in September, Ahmadinejad on three occasions made an unambiguous offer: He said Iran would stop all enrichment of uranium beyond the levels used in civilian power plants — if his country is able to buy specialized fuel enriched at 20 percent, for use in its research reactor that produces medical isotopes to treat cancer patients.

Obama should seize this proposal and send negotiators straightaway to hammer out specifics. Iran has been enriching uranium since 2006, and it has accumulated a stockpile of uranium enriched at up to 5 percent, sufficient after further enrichment for several nuclear bombs. Iran is also producing 20 percent material every day, and it announced in June that it planned to triple its output. Halting Iran’s current production of 20 percent material and its projected growth would be significant.

A stockpile of uranium enriched at 20 percent shrinks the potential timeline for breaking out to bomb material from months to weeks. In effect, having uranium enriched at 20 percent takes Iran 90 yards along the football field to bomb-grade material. Pushing it back below 5 percent would effectively move Tehran back to the 30-yard line — much farther from the goal of bomb-grade material. Even more important, extracting from Iran a commitment to a bright red line capping enrichment at 5 percent would stop the Islamic Republic from advancing on its current path to 60 percent enrichment and then 90 percent.

Stopping Iran from enriching beyond 5 percent is not, in itself, a “solution” to its nuclear threat. Nor was Reagan’s proposal to Gorbachev. The question for Reagan was whether we would be better off with the Berlin Wall or without it.

Iran today is the most sanctioned member of the United Nations; it has been the target of five Security Council resolutions since 2006 demanding that it suspend all uranium enrichment. The United States and Europe have organized their own, tougher economic sanctions forbidding businesses from trading with Iranian companies and limiting Iran’s access to financial markets.

But Iran does not require the permission of the United Nations or, for that matter, the United States to advance its nuclear program within its borders. Nor are current or future sanctions likely to dissuade Iran from progressing steadily toward a nuclear weapon.

So far, Obama has essentially continued the Bush administration’s policy toward Iran with one addition: an authentic offer from the start of his administration to begin negotiations. Negotiations, however, have not been feasible because of sharp divisions within Iran. Those rifts were exacerbated after the June 2009 elections, in which Iran’s ruling powers (Supreme Leader Ayatollah Ali Khamanei, Ahmadinejad and the Revolutionary Guard) rigged the presidential vote and then moved to suppress the opposition Green Movement protests. In the last two years, they have tightened control over their society.

Enter Ahmadinejad’s proposal to stop all enrichment at the 5 percent level — without preconditions. Although differences between Ahmadinejad and the supreme leader have become evident, the United States should pay attention to the president’s offer.

Arguments against testing the offer are easy to make. An embattled Ahmadinejad may not be able to deliver. Iran will use negotiations to seek to relax or escape current sanctions. If a deal were reached, it would be more difficult to win international support for the next round of sanctions. An agreement that stops only the 20 percent enrichment could imply a degree of acceptance of Iran’s ongoing enrichment up to 5 percent.

Recognizing all of these negatives, however, the policy question remains: Would the United States be better off with Iran enriching its uranium to 20 percent or without it?

President Obama should act now to test Ahmadinejad’s word.

Graham Allison is director of the Belfer Center for Science and International Affairs at Harvard’s Kennedy School of Government and the author of “Nuclear Terrorism: The Ultimate Preventable Catastrophe.”