I think the main effect if he invades later will be George Bush Sr.'s reelection. During the Gulf War Bush's popularity, if I'm remembering correctly, essentially reached its height. You delay the Gulf War, and then let the Gulf War in 1992 proceed on schedule, Bush's popularity reaches its height right before the election. Unless Bush does something really stupid, he gets a second term.
Now, because the Gulf War happens in 1992 and not 1991, there's a chance Democrats won't feel as threatened by Bush's popularity as they did historically. So Clinton's nomination might be butterflied away.