An open primary is one in which voters can take part in either the Democratic or Republican nominating contests regardless of their party affiliation. Voters registered with third-parties and independents are also allowed to take part in open primaries.

An open primary is the opposite of a closed primary, in which only registered members of that party can take part. In a closed primary, in other words, registered Republicans are allowed to vote only in the Republican primary, and registered Democrats are allowed to vote only in the Democratic primary.

Voters registered with third-parties and independents are not permitted to take part in closed primaries.

Support for Open Primaries

Supporters of the open primary system argue that it encourages voter participation and leads to greater turnout at the polls.

A growing segment of the U.S. population is not affiliated with either the Republican or Democratic parties, and is therefore blocked from taking part in closed presidential primaries.

Supporters also argue that holding an open primary leads to the nomination of more centrist and less ideologically pure candidates who have broad appeal.

Mischief in Open Primary States

Allowing voters of any party to take part in either the Republican or Democratic presidential primary often invites mischief, commonly referred to as party-crashing.

Party-crashing occurs when voters of one party support "the most polarizing candidate in the other party's primary to bolster the chances that it will nominate someone 'unelectable' to general election voters in November," according to the nonpartisan Center for Voting and Democracy in Maryland.

In the 2012 Republican primaries, for example, Democratic activists launched a somewhat organized effort to prolong the GOP nomination process by voting for Rick Santorum, an underdog, in states that held open primaries.

That effort, called Operation Hilarity, was organized by activist Markos Moulitsas Zuniga, the founder and publisher of Daily Kos, a popular blog among liberals and Democrats. "The longer this GOP primary drags on, the better the numbers for Team Blue," Moulitsas wrote.

In 2008, many Republicans voted for Hillary Clinton in the 2008 Democratic presidential primary because they felt she had less of a chance of defeating presumed Republican nominee John McCain, a U.S. senator from Arizona.

Open Primary States

Here is a list of states that held open presidential primaries or caucuses in 2012, according to the Center for Voting and Democracy:

• Alabama — Both Republican and Democratic primaries are open.
• Alaska — Democratic primary is open; Republican primary is open but party requires voters to be registered as a Republican 30 days before Election Day.
• Arkansas — Both Republican and Democratic primaries are open.
• Georgia — Democratic primary is open; Republican primary is open but party requires voters to "declare an oath of intent to affiliate with the particular party for whom they are voting on Election Day.
• Hawaii — Democratic primary is closed; Republican primary is open but voters are required to fill out a GOP registration on that day.
• Indiana — Both Republican and Democratic primaries are open, but participants must have voted for a majority of the nominees of the party holding the primary in the prior election.
• Michigan — Both Republican and Democratic primaries are open, but participants are required to vote for all one party and can't split their ballot after they enter the voting booth.
• Minnesota — Both Republican and Democratic primaries are open.
• Mississippi — Both Republican and Democratic primaries are open, but voters are expected to support that party's nominees in the election.
• Missouri — Both Republican and Democratic primaries are open.
• Montana — Both Republican and Democratic primaries are open.
• North Dakota — The Democratic primary is open; Republicans ask voters taking part in the primary to register with the GOP for the following election.
• Ohio — Both Republican and Democratic primaries are open.
• South Carolina — Both Republican and Democratic primaries are open.
• South Dakota — Democratic primary is open; Republican primary is closed.
• Tennessee — Both Republican and Democratic primaries are open.
• Texas — Both Republican and Democratic primaries are open.
• Utah — Democratic primary is open; Republican primary is closed.
• Vermont — Both Republican and Democratic primaries are open.
• Virginia — Both Republican and Democratic primaries are open.
• Wisconsin — Both Republican and Democratic primaries are open.