Assuming Paul loses, the Libertarian Party will have an easier task than usual: It will be able to concentrate its organizing among the people who voted for Paul in the primaries. That could easily amount to enough voters to deny Republicans a victory in the general election. (In other words, the libertarian candidate in this situation would be Ralph Nader in reverse.)
The winning Republican nominee would need Paul to campaign actively for him to prevent this scenario. But why wouldn’t Paul just go home to Kentucky to campaign for his own re-election? His Senate seat will be up in 2016.
Paul would then be the unusual politician who could actually secure some voters as the vice presidential nominee.