Who can vote?
Voters registered as Republicans can vote only in the Republican primary; Democrats can vote only in the Democratic primary.
People who registered as unaffiliated or independent voters can vote in either one. But those who want to vote in the Republican primary will have to fill out a new voter registration card at the polls and join the Republican party.
Read more here
Also see IndependentVoting.org and the Open Primaries page on The Hankster, and why not go national at OpenPrimaries.org!