WASHINGTON — Sen. Chuck Grassley, R-Iowa, was sworn in as president pro tempore of the Senate, Friday. The president pro tempore is a constitutional office customarily bestowed on the longest serving member of the Senate majority party. As president pro tempore, Grassley is now the second highest-ranking official in the Senate — behind the vice president, who serves as president of the Senate — and follows the vice president and speaker of the House in the line of presidential succession.