Updated: 12:58 p.m.

Senate Republican Conference Chairman Lamar Alexander (Tenn.) announced today that he is dropping his Whip bid and will step down from leadership in January, with GOP Policy Committee Chairman John Thune (S.D.) revealing shortly thereafter that he would seek the Conference chairman post.

In a letter sent to Republican Senators this morning, the Tennessean and close confidant of Minority Leader Mitch McConnell (R-Ky.) explained that he could have more of an effect on the issues that he cares about outside of leadership. Alexander, 71, said in the letter that he plans to run for re-election in 2014. Thune, while still keeping his options open to launch a Whip bid for the next Congress, told reporters that he would run for Conference chairman in January when Alexander steps down.

“Lamar leaves very big shoes to fill,” Thune told reporters. “I do think, come this [time] next year, we’re going to need to be out there articulating in a very clear way a vision for this country, and I want to help.”

With Thune seeking the Conference chairmanship, Sen. Mike Johanns (R-Neb.) announced he would drop his bid for that post and support his colleague from South Dakota. Johanns did not rule out running for another leadership position, but he declined to discuss what he might do. Conference Vice Chairman John Barrasso (Wyo.), however, announced he would run for Policy Committee chairman.

Alexander, the No. 3 Republican in leadership, was headed toward a battle for the Whip spot against National Republican Senatorial Committee Chairman John Cornyn (Texas). Both announced their plans to seek the post within hours of Senate Minority Whip Jon Kyl’s (Ariz.) February announcement that he would not seek re-election in 2012. As recent as Thursday of last week, in an interview with Roll Call, Alexander gave no indication that he was reconsidering his Whip campaign.

But Alexander said he has been giving thought for a while to dropping his bid and exiting leadership. Alexander’s decision to drop his bid could open the door for Thune or others in the Conference to challenge Cornyn for the No. 2 Whip position in November 2012, when leadership elections for the 113th Congress will be held.

“Next January, following our annual retreat, I will step down as chairman of the Senate Republican Conference. At that time, I will have completed four years or the equivalent of two two-year terms,” Alexander wrote in the letter to colleagues. “Stepping down from leadership will liberate me to spend more time working for results on the issues I care most about. I want to do more to make the Senate a more effective institution so that it can deal better with serious issues.”

“There are different ways to provide leadership within the Senate. After nine years here, this is how I believe I can now make my greatest contribution. For these same reasons I do not plan to seek a leadership position in the next Congress,” Alexander continued in the letter, in regard to running for Whip. “I said to Tennesseans when I first ran for the Senate that I would serve with conservative principles and an independent attitude. I will continue to serve in that same way. I am a very Republican Republican. I intend to be more, not less, in the thick of resolving serious issues. And I plan to run for re-election in 2014.”