Yes. An undeclared voter may vote in a state primary or a presidential primary. You will be required to choose either a Republican or Democratic ballot when you go to vote.
You may change your party affiliation with the clerk of the town or city where you are domiciled or at any scheduled meeting of the supervisors of the checklist except for during the period of time between the first day of the filing period for the primary election and the date of the primary election itself.
At this time, the official political parties in the State of New Hampshire are the Democratic and Republican parties.
“Party shall mean any political organization which at the preceding state general election received at least 4 percent of the total number of votes cast for any one of the following: the office of governor or the offices of United States Senators.” RSA 652:11