If the Iraqi insurgents defeat the U.S. then every bad guy on earth will study and learn their techniques. The people now running for president will find themselves in bigger heaps of trouble than the current one now is — trouble that this presidential campaign hasn’t even dealt with.
But, look, the Iraqi insurgency is hardly the first group to demonstrate that it's possible to force foreign occupying armies to withdraw from territory where they're not wanted even if the occupying army is, in some sense, militarily superior. This has been a well-known feature of the world for decades, if not centuries. Indeed, it's worth pointing out that advocates of invading Iraq used to be perfectly aware that we wouldn't be able to use military force to trump public opinion. Remember that "greeted as liberators" business? Remember when the administration was denying there even was an insurgency? That's said that stuff for a reason. The contention was that we wouldn't need to fight a counterinsurgency campaign, not that we were prepared to fight and win one.