Party Raiding is the term for members of one political party voting in another party's primary in an effort to either nominate a weaker candidate or at the very least prolong divided support between two or more contenders for that party's nomination (especially for President).

Party raiding can only occur in jurisdictions which allow open primary

Open primary

An open primary is a primary election that does not require voters to be affiliated with a political party in order to vote for partisan candidates. In a traditional open primary, voters may select one party's ballot and vote for that party's nomination. As in a closed primary, the highest voted...

 voting. As examples, Texas

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

 permits open primary voting (though a voter cannot vote in both primaries), while neighboing Oklahoma

Oklahoma

Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...

 operates under a closed primary system (with one catch: a state party can open its primary to independent voters at the sole discretion of the party chairman).

A notable example of attempted party raiding was Operation Chaos in the Democratic primary

Democratic Party (United States) presidential primaries, 2008

The 2008 Democratic presidential primaries were the selection process by which voters of the Democratic Party chose its nominee for President of the United States in the 2008 U.S. presidential election...

 in 2008 when Rush Limbaugh

Rush Limbaugh

Rush Hudson Limbaugh III is an American radio talk show host, conservative political commentator, and an opinion leader in American conservatism. He hosts The Rush Limbaugh Show which is aired throughout the U.S. on Premiere Radio Networks and is the highest-rated talk-radio program in the United...

 encouraged Republicans

Republican Party (United States)

The Republican Party is one of the two major contemporary political parties in the United States, along with the Democratic Party. Founded by anti-slavery expansion activists in 1854, it is often called the GOP . The party's platform generally reflects American conservatism in the U.S...

 to vote for Hillary Clinton in an effort to weaken Barack Obama

Barack Obama

Barack Hussein Obama II is the 44th and current President of the United States. He is the first African American to hold the office. Obama previously served as a United States Senator from Illinois, from January 2005 until he resigned following his victory in the 2008 presidential election.Born in...

politically.