If Romney wins, it screws the US for more than just the next four years. The Supreme Court nominations (as well as other federal judicial appointments) made by a Romney presidency will reach far into the future.
I reckon it'd have massive impacts on the diplomatic abilities of the US, too.
Of course it would, which would be awful in its own right. But the thing about the courts in the US is that they are an even more powerful force than they are here in the UK, and this could ultimately keep any progressive changes in legislation in the US from being realised. And then there are the other effects of rulings.
Basically, the Supreme Court is a branch of the American government. There are three: the legislature (House/Senate), the executive (the president), and the judicial (The Supreme Court and lower courts). There is a system of checks and balances (at least theoretically) which are meant to keep all three branches balanced, separate, and in check. The legislative branch makes the laws, the judicial branch interprets the laws, and the executive branch carries them out is the general short hand way of explaining it, but of course it's more complicated than that and the roles of the individual branches have evolved over time. Supreme Court justices, once appointed and confirmed sit until they retire, are impeached and convicted, or die. Sorry if this is something people know about already, but if you didn't, this is something you need to understand to grasp the gravity of another right wing president.
Right now there is a liberal (American sense) minority in the US Supreme Court. Four justices are over the age of 70. Of those, one is centrist liberal, one is centrist, one is solidly conservative, and the eldest is currently the most liberal. If Romney is able to replace two of these due to death or retirement he can alter the path the US takes for decades. If he appoints one that isn't from the conservative side (and remember, only one of the over 70s are conservative), he can do the same.
How is this important? They don't just rule on things like abortion rights or reading Miranda rights. They can shape how elections are run and won in the US. They decide whether a corporation should be treated as an individual person or not. They can strike down overreaching laws/executive actions like future versions of the Patriot Act. They can determine how the US treats its prison population and the rights someone who is not American hold. Ultimately, they are the last line of protection of the vulnerable against the state and their decisions can indirectly have a global reach.