Representative Sander M. Levin, Democrat of Michigan and the longest-serving Jewish member of the House, said something important this week: “In my view, the only anchors in public life are to dig deeply into the facts and consult broadly and then to say what you believe.”

His words were important for two reasons. First, they defied a prevalent political culture of ignoring inconvenient facts, consulting narrowly if at all, and never saying what you believe when it’s not what your constituency wants to hear. Second, his statement concerned Iran, an issue where fact-based reasoning on Capitol Hill and beyond tends to take second place to preposterous posturing — as per the Republican presidential candidate Mike Huckabee’s statement that the nuclear deal with Tehran would march Israelis “to the door of the oven.”

Levin’s reflection led him to the sober, accurate conclusion that the agreement was “the best way to achieve” the goal of preventing Iran from advancing toward a nuclear weapon, an outcome that would make Israel, the Middle East and the world “far more secure.” Not the ideal way, the perfect way, or a foolproof way, but, in the real world of ineradicable Iranian nuclear know-how, the best way attainable. That is also the view of other parties to the deal — the not insignificant or unserious powers of Russia, China, Britain, France and Germany.

Why? Levin, a longtime friend of Israel, was thorough. Because the accord, if fully implemented, slashes Iran’s stockpile of enriched uranium by 97 percent, prevents enrichment above 3.67 percent (a long way from bomb grade) for 15 years, intensifies international inspections exponentially, holds Iran at least a year from having enough material to produce a weapon (as opposed to the current two months), cuts off a plutonium route to a bomb, preserves all American options in combating Iranian support for Hezbollah, and is far better than an alternative scenario where international sanctions would fray and “support from even our best allies if we move to the military option would be less likely.”

Congress was given 60 days to review the deal. Sentiment is generally shoot-from-the-hip hostile. A resolution of disapproval that would be vetoed by President Obama is likely; the president probably has enough support to resist an override of his veto. But before following such an unsatisfactory path to assumption of a historic accord, members of Congress, including Senator Chuck Schumer, the normally outspoken New York Democrat who has discovered his inner reserve on this matter, should do their own version of Levin’s deep-dig questioning. They should also peruse a letter from five former United States ambassadors to Israel — including Thomas Pickering — and from former senior officials — including Nicholas Burns, under secretary of state for political affairs — that urges both chambers not to reject a deal without which “the risks will be much higher for the United States and Israel.”

Yes, the risks will be far greater. There is huge, if uncertain, upside potential to the establishment of an American relationship with Iran through this agreement. The downside potential in its absence is as great — and includes war.

It is intriguing that, along with Israel and Republican members of Congress, the most vociferous criticism of the deal has come from Saudi Arabia. The Saudis have had it with what they see as American fecklessness. They have been convinced since the Iraq invasion that the United States is pro-Shiite (read pro-Iran). They are so persuaded of Iran’s anti-Sunni imperial designs that they have embarked on an indiscriminate bombing campaign in Yemen with the purported aim of stopping the Houthis, seen as Iranian proxies.

Now the Saudis are American allies. Iran is, and will for the foreseeable future remain, a hostile power. But what have our “allies” done for us of late? Promoted, through madrasas and other means, the conservative Wahhabi Islam whose fierce anti-Western teachings provided the context for the emergence of Al Qaeda, the Taliban and, most recently, the Islamic State. Manipulated oil prices, most recently down, in order to undermine America’s liberating energy revolution through an attempt to make shale oil uncompetitive. Shunned Obama’s attempts to reassure Sunni monarchies that the Iran deal would not mean diminished support — and all this, of course, from the country that furnished the manpower for 9/11.

The Saudis are in lock step with Israel on hostility to the Iran deal but are no friends of Israel. Their goal, despite America’s dwindling dependence on the kingdom for oil, is to preserve a Middle Eastern status quo that limits American strategic options — including the possibility that Iran and the United States might find common cause in combating the Islamic State or, years from now, re-establish diplomatic relations.

Any deep dig into the facts, of Levin’s courageous kind, cannot escape the question of whether a deal with an enemy, Iran, so fiercely opposed by this one ally, Saudi Arabia, might not, over time, change the Middle East equation in ways favorable to America’s national interest.