Today, Iran will hold a presidential election that has brought voters to the streets nightly over the past week, young urban reformers clad in challenger Mir Hossein Mousavi's emblematic green, women with fingers dipped in green to show their allegiance, and backers of Iran's hard-line President, Mahmoud Ahmadinejad, dressed conservatively and wrapped in Iranian flags.

At stake in this election are many things, including economic policy, civil rights, and the tone of Iranian foreign policy.

In 2002, President Bush deemed Iran part of an "axis of evil"--and Iran has been a central focus of recent U.S. attention since, continuing America's long and complicated history with the country. If neither of the top candidates garners 50 percent of the vote, there will be a runoff on June 19, and we'll wait another week to find out who Iran's next president will be.

It's an election that's being watched by an international audience looking for insight into Muslim-world politics, with many observers hoping Mousavi's moderation will prevail, defusing the cultural and nuclear tension between Iran and the West.

So what does Iran's election mean for the U.S.? It all depends on what happens. Here are four outcomes, the last of which speaks to the question on everyone's mind: what if Mousavi wins?

1. Ahmadinejad wins--by a landslide. Reform politics in Iran have gained momentum in this election, and an Ahmadinejad landslide would be a setback. But it may not utterly crush moderate sentiment: Mousavi is reportedly popular among Iran's young--and 70 percent of Iran's population are under 30. So, just as there was momentum against President Bush in 2004 (a crushing election year for Democrats), that anti-Bush momentum continued in 2006 and culminated in President Obama's 2008 victory--and the same could happen in Iran.

For the Obama administration, it would mean begrudging engagement of Ahmadinejad.

The U.S. has committed to engaging Iran, but it has avoided acknowledging Ahmadinejad so far. Knowing he could be defeated in June, there was no reason to legitimize him in the eyes of Iran's voters. Obama's video message to Iran was seen as a way to circumvent the hard-line president and speak directly to the people--in fact undermining him as the election approached. Ahmadinejad has openly challenged Obama, calling on the U.S. to apologize for 60 years of "crimes" against Iran--and now the U.S. government would have to deal with him.

2. Ahmadinejad wins--and it's close. Analysts fear that, no matter the outcome, there will be some backlash after this election. If Ahmadinejad wins in a close contest, Mousavi's impassioned supporters may allege that it was stolen (some reformists reportedly think a 5 million-vote cushion is needed to offset irregularities and corrution). The U.S. could be faced with political turmoil in Iran, and it would have to choose carefully what to say about the election's results, especially if it believes Ahmadinejad won unfairly.

3. Mousavi wins--and it's close. To be clear, Iran's president is not in charge: the supreme leader, Ayatollah Kahamenei, will retain ultimate power in Iran no matter who wins today, though Mousavi, if he wins, will have influence. Iran's presidency has been likened to a strong U.S. vice president, a Cheney for instance, and if Mousavi wins in a close election, one question it might raise is: without a sweeping popular mandate, would his influence be less?

4. Mousavi wins--by a landslide. This is the prospect that has Western observers most excited. Again, a new president in Iran would not change everything--Khamenei will retain power no matter what. As a U.S. official told Foreign Policy's Laura Rozen, "there will not suddenly be flowers blooming" if Ahmadinejad loses. (See Rozen's piece for a good explanation of the election's dynamics in Iran, and how it relates to the U.S.)

But Mousavi has attacked Ahmadinejad for closing Iran off from the rest of the world. He's said he wants to talk to the P5+1 countries (permanent U.N. Security Council members plus Germany) about Iran's nuclear program. He has slammed Ahmadinejad for denying that the Holocaust happened. This is the type of leader the U.S. would want.

The power of the president, while certainly inferior to Khamenei's, is up for some interpretation. Khatami, Ahmadinejad's predecessor, was able to convince the supreme leader to stop uranium enrichment; Ahmadinejad was able to convince him to start it, University of South Florida Professor Mohsen Milani points out in an interview with the Council on Foreign Relations. So presidents are not without effect: they propose policies, and the Ayatollah decides on them, but presidents do have influence.

One thing analysts agree on is that tone is important, and Mousavi could set a new one. He'll be less confrontational, and his attitude could foster a new political environment--one that's much more friendly to the U.S., one that could push Iran away from isolation and open it up, for instance, to warmer relations with the U.S.

And then there's the broader theme of moderation vs. "Islamic extremism" and the U.S. struggle against the latter. If Mousavi wins, that's the frame in which many Western observers will place it.

"Looming large right now is a seeming trend throughout this war-torn part of the world away from Islamic extremism and toward more moderate politics. This is what occurred in recent elections in Pakistan and Iraq, and most notably two days ago in Lebanon," Council on Foreign Relations President Emeritus Leslie Gelb wrote in an op-ed this week.

Placed within that context, a sweeping victory for Mousavi would help set such a constructive, moderate tone, signaling a broad mandate for moderation. It would be seen as the next Muslim-world election in which voters turned away from the hard line. Obama would get a "new beginning" of sorts with Iran--at least a beginning of post-Ahmadinejad rhetoric.

Will Obama get credit for it if Mousavi wins? Some suggested his Cairo speech affected Lebanon's election, and he's been more directly involved in Iranian politics, thanks to the video message.

If Mousavi wins, Obama probably won't have had much to do with it, one Suzanne Maloney of Brookings says.

"[W]hatever the outcome, the 'Obama factor' is almost certainly a secondary one at best in the Iranian election. To the extent that Obama is relevant at all, it is simply because his expressed commitment to engagement and his early efforts to implement that strategy underline for many Iranians the potential opportunity costs of another Ahmadinejad term," Maloney, a former State Department policy advisor, says.

So the Obama effect may be dubious if Mousavi wins, but that doesn't mean Obama won't get some credit from pundits, and it doesn't mean the White House won't welcome the result.

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