If Iran declares itself to be nuclear, then at the very least Saudi Arabia and Egypt will sell their souls to get nuclear weapons. Arabs (Sunnis) can live in a Middle East where the Jews have nuclear weapons, but they can not live in a Middle East where both Jews and Persians (Iran is not Arab, it is Persian, different race and Shia) have nuclear weapons and they themselves don't have any.
Besides, America has given them a guarantee that if they do not get into an arms race and allow it to build military bases in Middle East, it will protect them against all enemies.
Now, it seems America has enough control over Jewish Nukes, but it does not have any control over Shia Nukes. If Iran gets them and America is not able to stop it, Arabs will understand that America is completely powerless and of no firther use to them. They will approach Russia-China to get some nukes for themselves and also because they seem to have some influence over Iran too.