Al Franken is a U.S. senator from Minnesota. The opinions expressed in this commentary are solely those of the author.

(CNN) After careful review, I have decided that I will vote in support of the agreement the United States and our international partners reached with Iran last month.

It's not a conclusion I came to lightly. Since the deal was announced, I've consulted with nuclear and sanctions experts inside and outside government; Obama administration officials, including Energy Secretary Ernest Moniz; ambassadors from the other countries that negotiated alongside us; advocates for Israel on both sides of the issue; my constituents in Minnesota; and, of course, my colleagues in the Senate.

Many have expressed reservations about the deal, and I share some of those reservations. It isn't a perfect agreement.

But it is a strong one. This agreement is, in my opinion, the most effective, realistic way to prevent Iran from getting a nuclear weapon anytime in the next 15 years. It does so by imposing a series of physical limits on Iran's nuclear program, especially its production of the fissile material it would require to make a bomb. The agreement's verification provisions are extremely strong: 24/7 monitoring of, and unfettered access to, Iran's nuclear sites and ongoing surveillance of Iran's nuclear supply chain.

That means: In order to make a nuclear weapon in the next 15 years, Iran would have to reconstruct every individual piece of the chain -- the mining, the milling, the production of centrifuges, and more -- separately and in secret. The regime would have to run the risk of any of these steps being detected by international inspectors or our own comprehensive intelligence efforts. It would risk losing everything it gained from the deal, and the re-imposition of sanctions.

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You don't have to trust the regime's intentions to understand the reality it would face: Attempting to cheat on this agreement would carry an overwhelming likelihood of getting caught -- and serious consequences if it does.

We'll still have work to do to diminish the threat Iran poses to our national security and the safety of our allies in the Middle East, beginning with Israel. As sanctions are lifted, the non-nuclear threat to the region may grow, and we'll need to bolster our support to regional counterweights such as Saudi Arabia, and increase our support of and cooperation with Israel, accordingly. And, of course, we'll need to maintain our terrorism-related sanctions, which are unaffected by the deal.

But there's no doubt in my mind that this deal represents a significant step forward for our national security.

It's worth noting that many of the restrictions in the deal expire after 15 years -- leading some to express concerns about what might happen in year 16.

There will still be major checks on Iran's nuclear program after that date, including continued heightened monitoring and permanent, specific prohibitions on several of the steps necessary to build a bomb. Iran must never, ever have a nuclear weapon -- and we will still have every option we currently have, up to and including the use of military force, to prevent that from happening.

But we also must begin now to make the case to the world that the danger posed by an Iranian nuclear weapon will not expire in 15 years -- and remind Iran that, should it begin to take worrisome steps, such as making highly enriched uranium as that date approaches, we stand ready to intervene.

Photos: Look inside Iran: How ordinary people live Photos: Look inside Iran: How ordinary people live Will Iran give up its nuclear ambitions? As its leaders try to hammer out a deal with the United States and five other leading world powers, CNN gets rare access to the Islamic state. It's the holy month of Ramadan, which means Muslims fast all day and feast at night. In Tehran, Iranians flock to markets after dark to break their fast. Hide Caption 1 of 12 Photos: Look inside Iran: How ordinary people live Nuts, fresh berries and other fruits are in high demand at this souk, or bazaar, after sunset when the Ramadan fast ends. Hide Caption 2 of 12 Photos: Look inside Iran: How ordinary people live Market stalls heave with fresh fruit in Tehran, aiming to satisfy observant Muslims who have been fasting all day. Hide Caption 3 of 12 Photos: Look inside Iran: How ordinary people live While the shops are full of fruit, other Iranian industries have it harder because of decades of international sanctions. At the Modern Technic factory in Tehran, workers mold auto parts by hand or with only basic machinery. Hide Caption 4 of 12 Photos: Look inside Iran: How ordinary people live Despite its name, the Modern Technic factory is much less automated than most modern auto plants. Most of the machinery is Iranian-made or comes from Russia. Hide Caption 5 of 12 Photos: Look inside Iran: How ordinary people live The Modern Technic auto parts factory still uses equipment made in the Soviet Union because sanctions make it difficult to get new machinery. Hide Caption 6 of 12 Photos: Look inside Iran: How ordinary people live Ayatollah Ruhollah Khomeini led Iran for a decade after the Islamic Revolution of 1979. Today, Iran has a shrine to honor him in Tehran. Hide Caption 7 of 12 Photos: Look inside Iran: How ordinary people live There's space for hundreds of worshippers inside the shrine to Khomeini, with copies of the Quran laid out for each of them. Hide Caption 8 of 12 Photos: Look inside Iran: How ordinary people live The caretakers of the shrine are known as khodam. Hide Caption 9 of 12 Photos: Look inside Iran: How ordinary people live The dome of the shrine over Khomeini's mausoleum. The exterior of the newly constructed dome is still under scaffolding, the final golden tiles being laid. Hide Caption 10 of 12 Photos: Look inside Iran: How ordinary people live Children play in the shrine while their mothers pray or pay their respects to the late ayatollah. Hide Caption 11 of 12 Photos: Look inside Iran: How ordinary people live The heart of the shrine is the tomb of Khomeini, who died in 1989 but is still revered by his followers. Hide Caption 12 of 12

That said, we don't know what the world will look like in 15 years. As long as this regime holds power, Iran will represent a dangerous threat to our security. But it's possible that, by 2031, Iran may no longer be controlled by hard-liners determined to harm our interests. More than 60% of Iran's population is under the age of 30. These young Iranians are increasingly well-educated and pro-American.

We don't know how this tension within Iran will work out. But we do know that backing out of a deal we've agreed to will only embolden the hard-liners who insist that America cannot be trusted.

Indeed, while critics have eagerly pointed out what they see as flaws in the deal, I have heard no persuasive arguments that there is a better alternative. All the alternatives I have heard about run the gamut from unrealistic to horrifying.

For example, some say that, should the Senate reject this agreement, we would be in position to negotiate a "better" one. But I've spoken to representatives of the five nations that helped broker the deal, and they agree that this simply wouldn't be the case.

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Instead, these diplomats have told me that we would not be able to come back to the bargaining table at all, and that the sanctions regime would likely erode or even fall apart, giving Iran's leaders more money and more leverage -- and diminishing both our moral authority throughout the world and our own leverage.

And, of course, Iran would be able to move forward on its nuclear program, endangering our interests in the region -- especially Israel -- and making it far more likely that we will find ourselves engaged in a military conflict there.

Some critics seem to lust after such a conflict , with one of my colleagues suggesting that we should simply attack Iran now, an exercise he believes would be quick and painless for the United States. But this is pure fantasy, at least according to what our security and intelligence experts tell us. And it's certainly not the lesson anyone should have learned from the disastrous invasion of Iraq.

In March, 47 of my Republican colleagues took the unprecedented step of sending a letter to Iran's leaders just as these sensitive negotiations were nearing an accord. It was a clear attempt to undermine American diplomacy -- and a signal that they would oppose any deal with Iran, no matter its terms.

It's not surprising that these critics now oppose the finished deal. But it is disappointing that they refuse to acknowledge, let alone take responsibility for, the dire consequences that would almost certainly result from killing it.

Diplomacy requires cooperation and compromise. You don't negotiate with your friends; you negotiate with your enemies. Indeed, no one who's for this deal has any delusions about the nature of the Iranian regime, any more than American presidents who made nuclear arms agreements with the Soviet Union had delusions about the nature of the communist regime there.

For a long time, it has looked like our only options when it came to Iran would be allowing it to have a nuclear bomb or having to bomb the country ourselves. This agreement represents a chance to break out of that no-win scenario.

And to take the extraordinary step of rejecting it -- because of clearly unrealistic expectations, because of a hunger to send Americans into another war, or, worst of all, because of petty partisanship -- would be a terrible mistake.