Doesn't Indiana have open primaries? If so, there's your answer.
Don't know the technicalities of it, but this is how Wikipedia defines it:
http://en.wikipedia.org/wiki/Primary_election
Open primary. A registered voter may vote in any party primary regardless of his own party affiliation. When voters do not register with a party before the primary, it is called a pick-a-party primary because the voter can select which party's primary he or she wishes to vote in on election day. Because of the open nature of this system, a practice known as raiding may occur. Raiding consists of voters of one party crossing over and voting in the primary of another party, effectively allowing a party to help choose its opposition's candidate. The theory is that opposing party members vote for the weakest candidate of the opposite party in order to give their own party the advantage in the general election. An example of this can be seen in the 1998 Vermont senatorial primary with the nomination of Fred Tuttle as the Republican candidate in the general election.
Semi-open. A registered voter need not publicly declare which political party's primary that they will vote in before entering the voting booth. When voters identify themselves to the election officials, they must request a party's specific ballot. Only one ballot is cast by each voter. In many states with semi-open primaries, election officials or poll workers from their respective parties record each voter's choice of party and provide access to this information. The primary difference between a semi-open and open primary system is the use of a party-specific ballot. In a semi-open primary, a public declaration in front of the election judges is made and a party-specific ballot given to the voter to cast. Certain states that use the open-primary format may print a single ballot and the voter must choose on the ballot itself which political party's candidates they will select for a contested office.
I'm a libertarian, but also a registered Republican so I can have a say in what goes on.
I'm sure many libertarians do this. Not necesarily pointing at you, Fletch, but I wonder what percentage of libertarians select a democrat primary ballot and how many select a republican.