It may be Russia that recently sent its troops across international borders in violation of the UN Charter in a "humanitarian intervention" to protect the South Ossetians (can the Russians get Samantha Power to do PR for them about the "Responsibility to Protect"?), but Iran, apparently, remains the bogey of choice for McCain and the neocons, as indicated by McCain's "tiny" ad. Apparently the neocons think Iran will work better to "scare the hell out of the American people," in the phrase attributed to Senator Vandenberg. Russia has one big demerit as a neocon bogey: it has too much ability to defend itself. It has nuclear weapons, a veto on the UN Security Council, huge energy reserves, easy ability to be disruptive to U.S. plans across a range of fronts. As any bully knows, "pick on someone your own size" is not advice generally followed by the successful bully. The most successful bullies will generally choose a bogey that has no reasonable prospect of significantly hurting them. And that points towards Iran.
Does Iran represent a "serious threat" to the United States of America, or even to Israel? It depends, certainly, on what you mean by "serious" and "threat." It is, of course, in the interests of McCain and the neocons to conflate two very different kinds of "threats": military threats and political challenges.
Iran, any honest and knowledgeable person would admit, does not represent, now or in the foreseeable future, a significant military threat to the United States or even to Israel. (If it did, Americans and Israelis should ask why we bother having such huge military budgets, and in the case of Israel, mandatory military service, if these things are so irrelevant.) Iran, as far as anyone outside knows, does not possess nuclear weapons, and is not in the process of acquiring any. The latter assertion in some sense cannot be proved: if Bush Administration officials are pushed, they will say that it doesn't matter if Iran is currently seeking to acquire nuclear weapons in the normal sense of actually acquiring them, because they are acquiring the capacity to enrich uranium, and the capacity to enrich uranium is very useful for building nuclear weapons. If you define "seeking nuclear weapons" this way - seeking to develop a capacity which would have the effect of making it easier for you to acquire nuclear weapons in the future, should you wish to do so - then indeed, Iran is seeking a nuclear weapon. But according to this standard, Brazil and many other countries we are not threatening to attack are also "seeking nuclear weapons," and there is no provision in any international law, treaty or agreement which would justify unilateral U.S. or Israeli action against Iran on the basis of this "threat."
Even if Iran were to actually acquire a nuclear weapon - an extremely distant prospect made more likely by U.S. and Israeli threats - it's far from obvious that this would make the United States or Israel less secure in a military sense. It would still be true, for the foreseeable future after that distant and unlikely prospect, that the United States and Israel would have an absolutely overwhelming military advantage against Iran in any military confrontation, and it would also be true that everyone would know that for Iran to use a nuclear weapon against Israel would be an act of suicide. And every honest and knowledgeable person admits that Iranian leaders, like leaders elsewhere, are overwhelmingly pragmatic and rational in their actions. They may make misjudgments, they may undertake provocative acts when they see it as being in their interests to do so, but like most people everywhere, they do not want to die, and they do not want to lose power.
The primary utility, from the Iranian point of view, of having a nuclear weapon would be that the United States and Israel would have to formally abandon the fantasy of attacking Iran. That, from the point of view of the neocons, is the "threat." They don't want Iran to become another Russia, a country that you can't easily push around. (Of course, if the United States would respect international law and the UN Charter - as it demands of Russia - then the United States would have to concede this, even if the Iranians only had sticks and stones.)
And the reason they see this as a high priority is that they see Iran as a political threat. If you see yourself as bully of the schoolyard, the presence of other bullies who aren't part of your gang is a latent threat. The next seemingly defenseless person you want to bully might turn to another bully for protection. And that's a threat to your power.
But it's far from obvious why the majority of the American people should see Iran in this way. The neocons were furious that Hizbollah was able to frustrate their plans to dominate Lebanon, in part because Hizbollah has patrons in the form of Iran and Syria. But what is the interest of the majority of Americans in dominating Lebanon? Arguably, the overwhelming majority of Lebanese, even those who have no love for Hizbollah, are far better off because the neocon project failed. A new national accord was reached in Lebanon, with the participation of all major factions, including Hizbollah and the US-backed coalition, and the support of all the regional patrons, including the Gulf countries, Iran, and Syria. An imperfect agreement, no doubt, but a far, far better outcome from the point of view of the interests of the majority of Lebanese than civil war.
The neocons are also furious that Iran has helped frustrate their fantasy to turn Iraq into a U.S. client state. This fantasy would almost surely have failed eventually, given its fundamental contradiction with Iraqi nationalism, even if Iran didn't exist. But there's no question that Iran has helped accelerate the failure of the neocon project for Iraq. So it's understandable that the neocons are angry. But should we be angry? Is it in the interests of the majority of Americans to try to make Iraq into a U.S. client state? It's certainly not in the interests of the U.S. soldiers who would have to be stationed there permanently, or their families, or American taxpayers who would have to foot the bill.
When the United States leaves Iraq, Iraq will not be a U.S. client state. Neither it be an Iranian client state, for the simple reason that this would not be in the interests of the majority of Iraqis, even the majority of Iraqi Shiites. It's in the interests of the majority of Iraqis to have good relations with Iran, but it's also in the interests of the majority of Iraqis to have good relations with Turkey and Jordan and the Gulf countries, and that means they have to balance the need for good relations with Iran with the need for good relations with others. And despite the last 30 years of national disaster, and the many real and deep conflicts that still have to be worked out, they still have an Iraqi national identity, and a strong desire to be free and independent actors in the world. No outside power, or coalition of outside powers, has the resources or stomach over the long-term to frustrate the desire of the majority of Iraqis to live in an independent country.
So if you're a neocon, Iran is a significant threat, not in a military sense, but in a political sense. But from the standpoint of the interests of the majority of Americans, Iran is not a significant threat, because the majority of Americans have no stake in the neocon project of dominating the Middle East. The majority of Americans, and the majority of Israelis and other peoples in the Middle East, would be far better served by serious attempts to assist in resolving the conflicts of the Middle East. The continuation of neocon policies of military adventurism in the White House, in the form of John McCain, represents a far greater danger than Iran to Americans and the world.