The endorsement is a key victory for Haley and represents mounting pressure on her runoff opponent, Rep. Gresham Barrett, who was also seeking McMaster’s endorsement.

Haley beat Barrett in the primary election Tuesday but didn’t secure the 50 percent she needed to avoid a runoff—she took home 49 percent to Barrett’s 22. McMaster came in third with 17 percent of the vote. Lt. Gov. Andre Bauer, who came in fourth, has already endorsed Barrett.