Senators Lindsey Graham and Bob Menendez are circulating a joint resolution that would, among other things, obligate the United States to assist Israel if it choose to start a war with Iran:
Urges that, if the Government of Israel is compelled to take military action in self-defense, the United States Government should stand with Israel and provide diplomatic, military, and economic support to the Government of Israel in its defense of its territory, people, and existence.
Hayes Brown clarifies what the bill would and would not mean:
The joint resolution is non-binding and would serve as neither a declaration of war nor an Authorization of the Use of Military Force like the near carte-blanche approval granted to President George W Bush at the onset of the Iraq War. It would, though, serve as an official announcement of U.S. policy to support any Israeli strike, whether the Obama administration had been previously consulted or not.
There are two big questions around the wording here. First, what amounts to "military" assistance? Does it obligate the United States to join Israel in an attack -- in effect, obligating the U.S. to go to war with Iran -- or resupply Israel after the fact?
The second question: what constitutes "self defense"?
In most plausible readings of how an Israeli strike would play it out, it wouldn't happen because Iran was about to launch an imminent nuclear attack but because Israel would feel, on balance, safer if Iran's nuclear facilities were destroyed. It would be a preventative war, self defense broadly defined, foreclosing the possibility that Israel could contain the threat from a nuclear Iran with deterrence.
These seem like a rather important ambiguities that should be resolved before the resolution moves forward.