Popular two-term Sen. John Thune (R-S.D.) is set to endorse Mitt Romney Wednesday morning in Iowa, the second major endorsement from the Senate for the former Massachusetts governor this week. "Mitt Romney has shown throughout his life in the private sector, as leader of the Olympics, as governor and in this campaign that he will not back down from difficult challenges," Thune said in a statement. "Washington could use these common sense principles at such a critical time."