LONDON — Is Hassan Rouhani, the new Iranian president, a game-changer? Initial indications leave open that possibility. Ignoring it would be foolish.

Gone, or tamed, is the inflammatory language, the anti-Western invective, the delusional accusations, the Holocaust denial and the Israel baiting that turned his predecessor, Mahmoud Ahmadinejad, into the villain from central casting. You do not have to be seduced by Rouhani’s apparent recent tweet wishing Jews a good Rosh Hashana — Israel’s Prime Minister Benjamin Netanyahu was “not impressed” — to perceive a significant change.

Ahmadinejad was a parochial rabble-rouser who proved to be all hat and no cattle. Rouhani, a Western-educated former nuclear negotiator, is a political pragmatist sensitive to the yearning of Iranians for an end to the nation’s pariah status and restoration of normality in its dealings with the world. He has promised to “pursue a policy of reconciliation,” impossible without compromise on Iran’s nuclear program.

There is every reason to be skeptical of Rouhani given past Iranian deception, the depth of mutual mistrust in U.S.-Iranian relations, and the decades-long investment in anti-American policy of the supreme leader, Ali Khamenei. But Rouhani’s opening should be tested rather than prejudiced through threats or the further sanctions Netanyahu is urging. Congress must hit “pause” on its restless urge to punish Iran.

During the Syrian crisis, Rouhani has been fierce in his condemnation of chemical weapons: Iran was attacked with them in the 1980-88 war by Saddam Hussein, who had the tacit backing of the United States. The Iranian president has been equally firm in his opposition to U.S. military intervention in Syria. His approach, and that of his foreign minister, Mohammad Javad Zarif, who apparently joined the president in wishing Jews a happy New Year, has been striking for its balance.

At the same time, Iran through its Revolutionary Guards has been a core supporter of Bashar al-Assad. Qassim Suleimani, a commander, has pledged support “to the end” for the Syrian regime.

With Iran there are always conflicting signals. Reading Alice in Wonderland is good preparation for dealing with it. But residents of Tehran report a palpable easing of tension under Rouhani. He bears dispassionate scrutiny.

It is a good sign that Secretary of State John Kerry, in noting that the “civilized world” had decided after World War I that chemical weapons should never be used again, chose to mention Iran first among the more than 180 countries that have signed the Chemical Weapons Convention.

Iran is in a vulnerable state: It has reached a post-revolutionary dead end where its anti-American rhetoric serves no strategic purpose. The ideological appeal of its Islamic theocracy in a changing Middle East is near zero. Supporting the brutal Assad has been strategically costly for itself and its Hezbollah surrogate.

Growing Sunni-Shiite tensions across the region can only hurt it. The sanctions-hit economy is a shambles. The country has leveraged its nuclear program for influence about as far it can without taking the added step to bomb-building that would invite a feared U.S. military response. Iran is stymied, its immense potential blocked.

But the Islamic Republic has demonstrated again a deep-seated resilience. By Syrian, Iraqi, Afghan and Egyptian standards it is an island of stability. As Rouhani’s election showed, it is capable of liberal eddies within its authoritarian model. Les Gelb, the president emeritus of the Council on Foreign Relations, noted this month that Iran’s elections are “more free and open than those in most Muslim countries.” This is a modest distinction — the 2009 election and its brutal aftermath were a fiasco — but underscores the important point that, unlike Assad’s Syria, Iran is not a totalitarian society.

The Islamic Republic is here to stay. It has largely acquired the nuclear know-how it sought without taking the high-risk bomb-making decision. The election of Rouhani reflects the desire of a society at an impasse to change course. All of which says: Negotiate now before another “red-line” drags the United States into confrontation.

This requires a new approach from the Obama administration. As William Luers, Thomas Pickering and Jim Walsh argue persuasively in The New York Review of Books, the United States “should take the initiative and communicate directly with the new leadership.” Coercive diplomacy, recommended by Obama’s former Iran hand Dennis Ross, is, they note, “an oxymoron” because “invariably the coercive side dominates the diplomatic side.” The goal, they say, should be a broad dialogue under which, in a phased process, Iran would agree to confine itself to limited enrichment for a peaceful nuclear program under strict international monitoring as sanctions were progressively lifted.

Rouhani and Obama will both be at the United Nations this month. They should meet. Beneath a stale enmity lurk many potential fields of cooperation. As Karim Sadjadpour of the Carnegie Endowment for International Peace has pointed out, “The collapse of the Assad regime would produce a common interest for Washington and Tehran in making sure that radical Sunni Islamists, who hate Shiite Iran even more than America, do not rule Damascus.”Far-fetched? Yes. But the swirling Middle East has nudged Tehran and Washington just a little closer.