The US military is now taking Israel's increasingly belligerent rhetoric against Iran more seriously and is directly warning it not to take any action. The delicate and volatile situation puts the United State's interests in the area in jeopardy and it is now calculating that confronting Israel and potentially offending its ally is the only way to prevent the situation from dangerously escalating. From the Guardian:
US military commanders have warned their Israeli counterparts that any action against Iran would severely limit the ability of American forces in the region to mount their own operations against the Iranian nuclear programme by cutting off vital logistical support from Gulf Arab allies.
US naval, air and ground forces are dependent for bases, refuelling and supplies on Gulf Arab rulers who are deeply concerned about the progress Iran has made in its nuclear programme, but also about the rising challenge to their regimes posed by the Arab spring and the galvanising impact on popular unrest of an Israeli attack on Iran.
The US Fifth Fleet is headquartered in Bahrain and the US air force has major bases in Qatar, Kuwait, Bahrain, the United Arab Emirates and Oman. Senior US officers believe the one case in which they could not rely fully on those bases for military operations against Iranian installations would be if Israel acted first.
I don't think the US has any intentions of attacking Iran, at least under an Obama administration, so it is using the argument that an early Israeli attack would hamper its own military plans as a way of appearing to aggressively oppose Iran while pursuing a more intelligent response to the threat it perceives. The pentagon understand that another war in the Middle East would dangerously overstretch the military and embroil the US in a potentially lethal conflict with no viable exit strategy. The aftermath of a regime changing assault on Iran would be awful beyond belief, and a power vacuum would open up that could pull the entire region into chaos.
It's clear that the US sees Iran as a threat to its interest in the region but it knows that it cannot pursue all out war against a country that 1. Has the ability to fight back, and 2. Wields serious influence in the region.
Under Netanyahu's extremist neo con government in Israel, there is little thought for the consequences of war with Iran. Netanyahu would already be in Iran had the US not stopped it, understanding that Israel's own survival is at stake. It marks a small but significant turning point in US foreign policy that it is now overtly telling Israel to back off. These small decisions make big impacts, furthering the argument that despite the ideological closeness of the US Presidential candidates, it does make a difference who is in power.
It is unclear whether Romney would commit to following Israel into a war with Iran, but his rhetoric suggests he would override his own military advisers and pursue Netanyahu's strategy of all out aggression. Romney could of course change his mind should he get into office (and he certainly has a track record of doing so), but it's probably not a bet the American public should make.