Sen. Thad Cochran won Tuesday's Mississippi Republican primary almost certainly without winning a majority of the Republicans who voted in the primary.
The victory by the 76-year-old incumbent over conservative challenger state Sen. Chris McDaniel, coupled with House Majority Leader Eric Cantor's loss earlier this months in his primary race, has focused new attention — and criticism — on so-called "open primaries," where independents and even the opposition party can have a say in picking a party's nominee.
Analysts and political scientists say the reported black and union voters who turned out for Mr. Cochran in the runoff victory in one sense showed the Cochran campaign understood how to play by the rules of Mississippi's election system.
"When you play a game, whether it be tennis, politics or football, you play according to the rules," said Dave Lublin, a government professor at American University. "Similarly in politics, the rules are set up in Mississippi so anyone can vote. They all knew the rules coming into it."
The outcomes of the Cochran and Cantor races have called into question the role of open primaries in present-day politics. Because they allow for more than the party's base to select nominees, experts say that this format benefits more centrist candidates, and tends to increase participation among unlikely or nontraditional voters, therefore leading to greater turnout at the polls.
In open primaries, voters of any affiliation may vote in the primaries of any party they choose so long as they only participate in one primary. Many open primary states don't require voters to indicate their partisan affiliation when registering to vote, according to the Center for Voting and Democracy.
Fourteen states, including Mississippi, have open primaries for congressional races, and 21 states have a mixed primary system where either Democrats or Republicans have an open primary while the other party's primary is limited only to registered party members.
"[The open primary system] seemed particularly helpful because Sen. Cochran realized he wasn't going to win outright, so he had to mobilize other voters," Mr. Lublin said. "There's something to be said about appealing across party lines."
But conservative and tea party groups who flocked to Mr. McDaniel's cause were fuming Wednesday at the result.
FreedomWorks for America President Matt Kibbe called Mr. Cochran's campaign efforts following the June 3 primary to woo non-GOP voters "disgraceful."
Critics say the open primary format can lead to mischief, with voters crossing party lines to vote for the weaker of the opposition's candidates, in hopes of raising their own party's chances in the general election.
But proponents of open primaries said they don't see the necessity of appealing to larger group of voters as necessarily a problem, and the open primary can give greater voice to voters not tied to either party.
Tim Hagle, an associate professor of political science at the University of Iowa, said independents, libertarians and third-party supporters particularly benefit in states that don't require voters to indicate an affiliation when they register to vote.
"Certainly to the extent for people who don't normally want to be affiliated with a party, it gives them an opportunity to pick the candidate," he said.
On the flip side, allowing anyone to vote in a party's primary may go against the very reason why political parties began holding primaries in the first place. Primaries formed as a way for party members to pick their nominee, as opposed to conventions or closed conclaves where "big wigs" picked the candidates in a back room.
"[We] moved to a primary system so that more people in that party have a say in who they want the nominee to be," Mr. Hagle said. "If they're an organized party, they don't really want people who aren't part of their party, who don't believe in the same basic principles, picking their nominee."
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