Difference between open primary and closed primary?

Answer 1

An open primary is a primary election (an election to select a candidate for a political party, or label, to stand in a later election) in which every registered voter is permitted to participate, even if he or she is not affiliated to that political party. In the United States, 18 states have open primaries (17 in Presidential elections, Hawaii being the exception). In Arizona and Massachusetts, unaffiliated voters can vote in primaries, but affiliated voters can only vote in the relevant party primary (e.g., a registered Republican cannot vote in the Democratic primary). In California, political parties have the option to allow or deny unaffiliated voters to participate in their primaries at a presidential level.

In Washington, Louisiana and California most non-Presidential elections use a two-round system, called a non-partisan blanket primary in the US. Under this system, candidates from every party participate in the primary election, with the two most popular candidates going to a run-off vote on the date of the general election. This can lead to a situation whereby, for example, two Republicans or two Democrats are against each other, or where only one of the two - or even neither - of the major parties qualifies for the second round.

The alternative is a closed primary. In a closed primary, only party members or affiliated voters are entitled to vote (e.g., only registered Republicans can vote in Republican closed primaries in the US). This is the most common method of primary around the world - another prominent example of the closed primary is in the United Kingdom, where constituency branches of the three major political parties select parliamentary candidates in 632 closed primary races each. They are more common in countries like the UK where political parties are more cohesive and united ideologically than in the US, where parties tend to be loose camps of generally like-minded individuals.

Answer 2

closed primary: Only the declared members of a party are allowed to vote for that party's nominees.

open primary: Voters do not need to declare their party preference in order to vote for the party's nominees.

Answer 3

Generally speaking a closed primary election only allows registered voters of a party to vote on the selection of a candidate from their own party. An open primary allows all eligible voters to vote for candidates.