SIOUX FALLS (AP) – South Dakota U.S. Sen. John Thune has climbed to the second-highest rung on the Senate Republican leadership ladder with a promotion to majority whip.

Thune had served in the No. 3 leadership post as chairman of the Senate Republican Conference since 2012.

A term limit for majority whips forced Texas Sen. John Cornyn out of the job. Thune was unopposed for the position and was elected by his Senate colleagues .

Whips are responsible for counting heads and rounding up party members for votes and quorum calls, and they occasionally stand in for the majority or minority leaders in their absence. The majority whip ranks behind only the majority leader among the leadership positions to which senators are elected by their colleagues.

The term “whip” is a reference to the member of a British hunting team who would ensure the dogs did not stray from the rest of the group. Thune acknowledged that keeping the narrow Republican majority together on controversial votes can be a challenge.

“When you’ve got a conference that has people on different ends of the spectrum like Ted Cruz and Susan Collins, it creates challenges,” he said, noting the differences between the conservative Texan Cruz and the moderate Collins, who is from Maine.

Thune will relinquish his chairmanship of the Senate Committee on Commerce, Science and Transportation to serve as whip.

Thune served in the U.S. House from 1997 to 2003 before beginning his service in the U.S. Senate in 2005. He is currently serving his third term.