I've not looked into Ron Paul's record on immigration, but a Libertarian running for president as a Republican is not electable in the general election. Thus I must decline to support him in the primary.
The same has been said of conservatives, ever since the 1964 Goldwater debacle.
Reagan demonstrated the falsity of that proposition in his election landslides of '80 and '84.
Eventually, the stigma of holding personal liberty as the highest moral value will subside and even Republicans may consider nominating a (small "l") libertarian for the White House.