The 'what if' factor is very high. Israel has been very patient with us and W has been fair to them (when he's kept the State Dept out of it). Consider the Israeli options compared to Iraq.
With Iraq, there was only one target of concern. Iran has quite an extensive and widespread nuke development infrastructure that can't be taken out by one strike.
Iraq was closer, though stretching Israeli tactics to reach it (in flight refueling), and didn't expect it. Iran is logistically more difficult for Israel to reach without using it's ballastic missiles AND they are expecting it.
Finally, potential US support/concurrence. A pre-election strike could cut both ways. However, it would probably hurt us more than help. I don't think we have enough ground troops to deal with any after effects. Iran has 12 divisions ready to cross into Iraq and our forces are dispersed in counter-insurgency ops. If Iran uses a strike as an excuse to go back into S. Iraq as they did during the Iran/Iraq war, things would get really messy and we'd have a good portion of our supply line cut off, not to mention that the south is where the Brits and other allies are working.