At the height of the Cold War, the Soviet Union had thousands of nuclear warheads aimed at American cities, and the Soviets were subject to numerous arms controls agreements. In an ideal world, the Soviet Union would have agreed to more severe nuclear constraints than those agreed by Presidents Kennedy, Nixon, Ford, Carter, Reagan and Bush and halted its meddling around the world.

But, as all of these presidents – Democratic and Republican – understood, holding out for the impossible is a recipe for no progress at all. Congress should take the same approach to the Iran nuclear deal.

Arms control agreements are rarely finished absolutes. Inevitably, their success depends on many factors that play out after the agreement is signed, including the political will of the parties to ensure verification and enforcement.

The key questions regarding the Iran agreement are: Will it stop Iran from obtaining a nuclear weapon? What are the risks of going forward with this agreement? And what are the risks if Congress rejects the agreement?

The plus-sides are clear. It includes severe restrictions on uranium enrichment and plutonium production, required transparency into Iranian activities and inspection provisions to assure that Iran’s nuclear program is, and remains, peaceful. The agreement will help deter Iranian cheating and provide the means to detect violations in time to take strong action.

Critics suggest that Iran would have agreed to entirely dismantle its nuclear enrichment facilities and stop all activities related to its civil nuclear program if only the U.S. had been tougher in negotiations. But had the U.S. taken such an approach in the early 1990s, we would not have encouraged and helped Russia, Ukraine, Kazakhstan and Belarus safely accelerate the destruction of their weapons and materials of mass destruction, and the risk of accidents or catastrophic terrorism would have been far higher over the past 20 years.

Although there are no absolute guarantees, nor can there be in diplomatic accords, our bottom line is that this agreement makes it far less likely that the Iranians will acquire a nuclear weapon over the next 15 years.

As with other agreements, Congress must recognize that there is no such thing as “perfect” verification. What is crucial is whether “effective” verification can be achieved. Can cheating be detected in time to take action before Iran could achieve a militarily significant advance? We believe the answer to that question is yes. Congress must listen carefully to our intelligence community and the IAEA’s views on any possible weaknesses in the verification regime, and then work with these entities to mitigate such vulnerabilities.

Opponents assert that sanctions relief would provide Iran with additional resources that would enable it to intensify its destabilizing behavior. This is a risk, but the argument that this risk can be avoided or reduced by the defeat of this agreement rests on a patently false assumption. Anyone believing that the present effective economic sanctions will be continued by other nations if Congress rejects this agreement is in a dream world. This agreement and the alliance that brought Iran to the negotiating table have focused on Iran’s nuclear activities, not its regional behavior, though both are serious dangers. This economics-sanctions alliance could never have been brought together to pursue a regional agenda on which alliance partners themselves strongly disagree.

With or without this agreement, the U.S. must intensify our efforts with other partners to counter Iran’s destabilizing regional activities and strengthen our cooperation with Israel and the Gulf States. If this agreement is rejected, both of these objectives become more difficult.

Finally, Congress must think long and hard about the consequences if this agreement is turned down. There is no escaping the conclusion that there will be grave implications for U.S. security and leadership in the decades ahead. Sanctions allies will go their own way, reducing the effectiveness of our financial tools and leaving Iran in a stronger position. Any future effort by this president, or the next, to assemble a “sanctions coalition” relating to Iran or other security challenges will be weakened. U.S. leadership and credibility, including efforts to achieve support for possible military action against Iran, will be severely damaged.

If the Iran agreement is upheld by Congress, the hard work of monitoring and enforcement is just beginning. This Congress and future Congresses, as well as future presidents, have a large and continuing role to play if “stopping the Iranian bomb” is to become a reality.

These crucial September votes will require members to search their own consciences. Whether they vote “yea” or “nay,” they must first look in the mirror and ask whether they are putting our nation’s interest first.

Our conviction is that this agreement represents our best chance to stop an Iranian bomb without another war in the Middle East.