Party raiding describes a tactic in American politics where members of one party vote in another party's primary election in an effort to either nominate a weaker candidate or prolong divided support between two or more contenders for that party's nomination (especially for president).
Party raiding can easily occur in jurisdictions which allow open primary voting. As examples, Texas permits open primary voting (though a voter cannot vote in both primaries), while neighboring Oklahoma operates under a closed primary system (although a state party may open its primary to independent voters at the sole discretion of the party chairman). In a closed primary state, party raiding entails crossover registration.
A notable example of attempted party raiding was Operation Chaos in the Democratic primary in 2008 when Rush Limbaugh encouraged Republicans to vote for Hillary Clinton in an effort to weaken Barack Obama politically. The effort was ultimately unsuccessful. Another occurred in the 2012 Republican primaries, where many Democratic voters in Michigan voted for weaker GOP candidate Rick Santorum over front-runner Mitt Romney in order to disrupt his campaign. This attempt was also unsuccessful.