A new poll of 20 nations, with 62 percent of the world’s population, finds that, among global leaders, President Obama inspires the most confidence — while the leaders of Russia and Iran inspire the least.

Nearly two-thirds of those polled by World Public Opinion (www.worldpublicopinion.org) had confidence in Obama. On the other hand, Iranian President Mahmoud Ahmadinejad took last place, while Russian Prime Minister Vladimir Putin came in next to last.

Yet global approval ratings don’t easily translate into policy results on Iran or Russia. That was apparent during Obama’s Moscow trip this week.

The bearded visage of Ahmadinejad hovered like a surly ghost over the president’s visit to Russia. Obama had hoped to convince Russia’s leaders and public of our common interest in preventing nuclear proliferation and defeating violent extremists.

To be more precise: Facing threats from Islamists in the country’s southern regions, Moscow should be worried about a nuclear Iran on its border. And if Iran refuses to halt its march toward nuclear weapons, Russia should support tougher U.N. sanctions against Tehran.

Obama made his pitch in a speech to university students at the prestigious New Economic School. If Washington and Moscow don’t cooperate, he told them, global barriers to nuclear proliferation will crumble. He insisted a controversial U.S. program to build missile defenses was aimed at Iran, not at Russia — and urged Russia to take part in the program.

Yet there was no evidence that Putin — or even the more modern Russian president, Dmitry Medvedev — got the message or was willing, for example, to rescind the sale of a strategic air defense system to Tehran.

Obama’s speech, which he hoped would reach a broad Russian audience, wasn’t even carried on the state-controlled TV networks, which are where most Russians get their news.

Indeed, Russia was quick to recognize the tainted “victory” of Ahmadinejad. The state-controlled media compared the protests of the Iranian opposition to the “color revolutions” in Ukraine and Georgia, which Putin has decried.

Some Russian pundits speculate that Putin prefers tensions between Tehran and Washington so oil revenues will remain high; Moscow needs the money. Whatever their rationale, Russian leaders still seem more interested in poking Washington in the eye than in curbing the proliferation threat.

So, for now, Obama’s chances of eliciting Russian cooperation in dealing with Tehran appear minimal. And without that cooperation, tougher U.N. sanctions may not be possible.

At the same time, the evolving political crisis in Iran has undercut any prospects for Obama’s policy of “engagement” with Tehran. Although the regime has forcibly squelched mass demonstrations, internal divisions within the elite are still playing themselves out.

In these circumstances, it’s hard to discern whom one would talk with, and to what purpose. The hard-line paramilitary Revolutionary Guards, loyal to Ayatollah Ali Khamenei, have clearly gained in power and seem to have little incentive to compromise at home or abroad.

A senior U.S. defense official recently told me that, in the spring of 2008, the commander of the Quds force, a secretive unit of the Revolutionary Guards, sent a message to Gen. David Petraeus. The message said Petraeus should know that “I, Brigadier General Qassim Suleimani, control the foreign policy of my country with respect to Iraq, Afghanistan, Lebanon, and Gaza.” In other words, forget the Iranian foreign ministry, Ahmadinejad, or even the supreme leader — the Quds force controls much of Iranian foreign policy.

And no one is entirely certain, following recent events, who controls Iran’s nuclear portfolio.

With Obama’s engagement policy on ice and tougher sanctions hard to envision, Iran analysts got a jolt on Sunday, when Vice President Biden seemed to say on ABC’s “This Week” that America would give Israel the green light to attack Iran’s nuclear installations.

Tuesday, in Moscow, Obama walked Biden’s remarks back. The United States is “absolutely not” giving Israel a green light to attack Iran, he told CNN. Glad to hear that. I think Biden got it right, however, when he said, “We’re not rushing to sit down (with Tehran).”

This is a time for Obama to breathe deeply, watch developments in Tehran, and keep pressing Russia to act in its own best interest on Iran. And to start thinking, if all else fails, about a Plan B.