Armin Laschet, the CDU's leader since January, is stepping down after leading the two-party Union bloc, which the CDU dominates, to its worst-ever election result in September. It was narrowly beaten by the center-left Social Democrats of new Chancellor Olaf Scholz.
The party decided to hold a ballot of its entirely roughly 400,000-strong membership, a first for the CDU. It says that nearly two-thirds of members took part.
There are three contenders, and if none wins more than 50per cent of the vote a runoff ballot is planned. That would be completed in mid-January, and a party congress Jan. 21-22 would officially sign off on the result.
Friedrich Merz, a prominent conservative, is making his third bid for the leadership.
He faces two more centrist candidates. Norbert Roettgen, a former environment minister who has chaired parliament's foreign policy committee in recent years, is making his second bid. And Helge Braun, who was Merkel's chief of staff from 2018 until she left office last week, is making his first attempt.
Merz lost by fairly narrow margins at party conventions in 2018 and in January, first to Annegret Kramp-Karrenbauer and then to Laschet. Roettgen was third in the contest in January.
Merkel gave up the party leadership in 2018 and announced that she wouldn't seek a fifth term as chancellor.
-With PTI inputs