Horst Seehofer has headed the CSU and been State premier of Bavaria since 2008.
Bavaria's motto is "laptops and lederhosen," as displayed here by the Christian Social Union's long-time leader, Edmund Stoiber.
Compared with Germany's other states, Bavaria saw the most refugees crossing its borders two years ago fleeing war and poverty. Chancellor Merkel's open-door policy lead to a lasting disagreement over how best to handle the crisis and whether to cap the number of asylum seekers in the country.
In Bavaria, a series of four attacks in close succession in summer 2016 fueled fears of terror.
But other topics have caused trouble between Ms. Merkel and Mr. Seehofer too, such as his meeting in 2016 with Russian President Vladimir Putin to discuss Ukraine – without the chancellor.
"There must never be a democratically legitimated party to the right of the CSU,” said leader Franz Josef Strauß; but with the Alternative for Germany, things are changing.