The U.S. Supreme Court turned aside a challenge by small political parties Tuesday to California’s “top-two” primary system, which requires all candidates to run in a single open primary election and moves the two top vote-getters, regardless of party label, into a November runoff.

A 2010 ballot measure, approved by 54 percent of the voters, established the new system in place of party primaries for all state and federal offices except the presidency. It was backed by business groups, which prefer more centrist candidates from both major parties, particularly the Democrats, because the more moderate candidate in a heavily Democratic district would presumably pick up Republican voters.