First, we need to understand the very real threat that Pakistan poses to world peace; much more so, in fact, than Iran ever did. Pakistan is a Sunni Muslim country that (as Romney pointed out) has 100+ nuclear weapons. Unfortunately, Pakistan also has a relatively weak and wildly unpopular government, and the Pakistani government's alliance with America has made it all the more unpopular. According to Mother Jones magazine, the Obama administration is well aware of the situation:
At the heart of this acerbic relationship, however, is Pakistan's arsenal of 110 nuclear bombs which, if the country were to disintegrate, could fall into the hands of Islamist militants, possibly from inside its own security establishment. As Barack Obama confided to his aides, this remains his worst foreign-policy nightmare, despite the decision of the US Army to train a commando unit to retrieve Pakistan's nukes, should extremists seize some of them or materials to produce a "dirty bomb" themselves.
Pakistan's military high command fears the Pentagon's contingency plans to seize its nukes. Following the clandestine strike by US SEALs that killed Osama bin Laden in Abbottabad in May 2011, it loaded elements of its nuclear arsenal onto trucks, which rumbled around the country to frustrate any possible American attempt to grab its most prized possessions. When Senator John Kerry arrived in Islamabad to calm frayed nerves following Bin Laden's assassination, high Pakistani officials insisted on a written US promise not to raid their nuclear arsenal. He snubbed the demand.
Nuclear weapons. On trucks. In a third-world country. With a wildly unpopular government. And a population that largely hates America and has largely neither heard of nor cares about 9/11 or why we wanted so badly to kill Osama bin Laden and to destroy al-Qaeda. Add to that particularly dangerous mix the drone strikes that the Obama administration has used to decimate the al-Qaeda leadership, but at the cost of many innocent Pakistani dead. These drone strikes are quite popular among most Americans; after all, we're getting the bad guys without putting our own military at risk, but America's use of drones that have killed innocent Pakistani men, women, and children in sovereign Pakistani territory has made us all the more hated in the eyes of the people of the nuclear-armed Pakistani nation. The prospect of driving Pakistan into the arms of extremists is something that American politicians, whether Democratic or Republican, want desperately to avoid. As the reference above points out, a senior Pakistan military officer recently stated, "We developed all these nukes to use against India. Now they turn out to be very useful in dealing with the US."