Mordor has just been added as the 51st state in the united states. when people go to vote in presidential primaries in mordor, they get to decide on the day of the election if they are going to vote in the republican or democratic primary. this is an example of what kind of primary?

Open Primary

Explanation:

One main characteristic of the open primary is that Voters do not have to be affiliated to a certain party in order to choose candidates from that party.

We can see this in this part of the excerpt:

.  .. they get to decide on the day of the election if they are going to vote in the Republican or Democratic primary. . .

Since you can votes regardless of political affiliation, Open primary is the time where voters can choose based on their personal beliefs, preferred policies, or how the candidates' opinion on a certain issue.

This primary tend to attract the largest numbers of moderates and bipartisan. Who want to focus on the individual representatives rather than the whole party platform.

try something like republic or democracy joined or something because its talking about a election party.