Sen. Clinton prefers diplomacy, but will not rule out military action. She has made contradictory statements regarding use of tactical nuclear weapons: in 2006 interview, said she would not use them; in 2007 debate, said she would not take option off the table.
Sen. McCain has called for greater diplomatic pressure and UN sanctions to deter Iran’s nuclear ambitions, but has also not ruled out use of the military, including tactical nuclear weapons, to prevent Iran from obtaining nuclear weapons, which he calls the “tripwire” and “the only thing worse than the U.S. exercising the military option.”
Sen. Obama says he would utilize diplomacy, sanctions, and (as a last resort) air strikes to prevent Iran from obtaining nuclear capabilities, but believes the U.S. military is too overstretched to consider ground troops against Iran while still in Iraq.
Point to Obama, because the military just can't afford to go anywhere while it's still in Iraq and Afghanistan. Back in '03, Iran was a bigger threat than Iraq. Five years later, Iran still has nuclear ambitions, but diplomacy is the only way we can deal with them.