Primary elections are held to determine a political party’s candidate for a general election. In the United States, primaries generally fall into three categories: open, closed and semi-closed. An open primary system does not require a voter to be registered with a certain political party in order to vote for partisan candidates. Instead, voters of any affiliation can vote in the primaries of any party, although they are technically forbidden from participating in more than one of those elections. According to the Center for Voting and Democracy, 21 states held open primary elections for presidential candidates as of 2012.

States with Open Primaries

The following states held open presidential primaries or caucuses in 2012: Alabama, Alaska, Arkansas, Georgia, Hawaii, Indiana, Michigan, Minnesota, Mississippi, Missouri, Montana, North Dakota, Ohio, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia and Wisconsin. Critics of the open primary system claims it allows members of one political party to vote in their rival party’s contest in the hopes of encouraging the nomination of the weakest candidate – a process known as “party-crashing.” However, there is no evidence to prove such a practice has ever affected the results of an election.

Examples of 'Party Crashing'

The 2012 presidential primary process resulted in one notable example of “party-crashing.” Democratic activists launched an effort called “Operation Hilarity” that aimed to prolong the Republican primary elections by encouraging Democrats in open primary states to vote for GOP underdog candidate Rick Santorum. In 2008, conservative talk show host Rush Limbaugh similarly encouraged Republican loyalists to disrupt the Democratic elections in open primary states, a plan he dubbed “Operation Chaos.” Neither political effort had a noticeable impact on either the 2008 or 2012 primary races.

States Where Primaries Vary By Party

Some states allow political parties to determine whether non-registered voters can vote in their primary elections. As a result, some states contain hybrid primaries, where one party may hold an open election and the other a closed. In both Alaska and Utah, for instance, the Democratic parties held open primaries in 2012, while their Republican counterparts limited primary participation to members of their own party. Idaho, Kansas and South Dakota similarly allow political parties to choose their own primary process.

'Top Two' Primary

Sometimes included in the “open primary” category, the top-two primary system puts all candidates – regardless of political affiliation – on the same ballot. The top two candidates who receive the most votes then face each other in the general election, essentially eliminating the party primaries altogether. The top-two system is used in California -- not including presidential races -- and Washington for state and federal offices. While some proponents of the top-two system have argued it could decrease political polarization by churning out moderate candidates, analysis' of those elections have found moderate candidates fared no better than in traditional open and closed primary races.