FREEPORT — If it works, the agreement reached in July among Iran, the United States, the United Kingdom, France, Germany, China and Russia will prevent Iran from engaging in nuclear research and development for 10 years, and will control the level of uranium enrichment for an additional five years.

In return for putting nuclear development on hold, economic sanctions on Iran will be lifted once it has complied with its commitments under the agreement, and $100 billion of frozen financial assets will be released.

Susan Collins of Maine and every other Republican in the U.S. Senate opposed the agreement. They were disappointed that it only postpones and does not permanently halt Iran’s nuclear development.

They think that the monitoring process is inadequate; that Iran will cheat and find ways to continue nuclear research; and that at least some of the $100 billion of released assets will be used to buy conventional weapons and to foment more chaos in the Middle East and elsewhere.

They wanted the deal to be renegotiated to extend the time nuclear development is halted, and to strengthen the inspection process.

Independent Maine Sen. Angus King understands that there are many uncertainties and risks embedded in the agreement, but believes that keeping Iran nuclear-free for 15 years, and perhaps longer, is worth the risks. He and other supporters believe that the monitoring process is adequate to detect significant infringement, and that sanctions can be re-imposed if Iran breaks the agreement.

The reality is that once sanctions are lifted, it will be very difficult to re-impose them. Europe badly wants Iranian oil, and there is already a long line of Western companies that are eager to establish commercial ties with a country of 81 million consumers. There would unlikely be much “snap” in the agreement’s snap-back provision for sanctions.

A renegotiation of the agreement that opponents want is also highly unlikely. Iran has said that it will abandon the deal and resume nuclear development if any attempt is made to alter it, and its stance throughout the negotiations gives little reason to doubt its resolve on the issue.

Ironically, the promise and benefits of economic integration and the desire of at least some of Iran’s leaders to achieve legitimacy and respect for their country on the world stage are powerful incentives for Iran to stick to the agreement.

Besides the uncertainties about whether the deal will work as advertised, there is another question. On a policy initiative of such importance, is it possible that every Republican in the Senate did the same careful analysis and came to the same conclusion about the deal’s merits? It’s possible, but another factor is at work here.

Ever since he was unable to capture a single Republican vote for the Affordable Care Act, President Obama has been dismissive of Republican policy concerns. Human nature being what it is, many Republicans have reacted to the president’s Olympian disdain by reflexively opposing him, even in cases where he might have had their support on policy merits.

Historically, all of our country’s most important foreign policy agreements have been in the form of treaties that have had substantial bipartisan support.

Obama didn’t think he could get 67 Senate votes for the Iran deal as a treaty, so it is structured as an executive agreement. The result is that although 54 Republicans and four Democrats voted for a resolution disapproving the deal, 44 senators, including King, were able to invoke a filibuster procedure that kept the resolution from going anywhere.

This may be fine for those who support both the president and the Iran deal, but domestic and foreign policies that have zero bipartisan support are likely to be fragile and subject to recurring challenge (think Obamacare).

In the case of the Iran agreement, the risk is that a future Republican president will disavow the deal or try to renegotiate it, and as a result the Iranians will resume their nuclear work. If sanctions can’t be re-imposed, that would leave use of military force as the only option to stop it, an option that would have no support in China or Russia, and very little anywhere else.

An agreement that halts Iran’s nuclear progress even temporarily deserves a chance and more support than it has gotten. It may fail because hardline Iranians attempt to cheat. But it is just as likely to unravel because of the lack of bipartisan support in the United States, another result of the most divisive presidency in modern times.