If the Israelis really are intent on attacking the Iranian nuclear facilities, they’re likely to do so before this November’s American presidential elections. If they started an attack and needed U.S. firepower to help them complete the task, Barack Obama might open himself up to perilous political attacks—for being indecisive, weak, appeasing, anti-Israel, you name it—if he didn’t follow through. It could cost him the votes of crucial constituencies. If the Israelis tried to pressure the United States into joining an attack after the election, Obama would have (to borrow a phrase from another context) more flexibility. So, to the extent the Israeli leaders have decided to attack (and it’s not at all clear they have), they are probably thinking: much better sooner than later.
Note that this simply implies that a foreign government would be relying on US domestic pressure to force the US administration to join a war it did not seek. I'm not sure what that is, but "alliance" is not the correct word.