At stake is the leadership of Europe's biggest economy in the era after Merkel, who has vowed not to run again after becoming Europe's predominant leader since taking office in 2005 and proving a winner with German voters.
The new CDU leader will be elected by 1,001 delegates at a digital congress. By tradition, the leader is usually - though not always - the chancellor candidate for the CDU's "Union" with its Bavarian sister party, the Christian Social Union (CSU).
Centrist Armin Laschet, arch-conservative Friedrich Merz and foreign policy expert Norbert Roettgen are vying for the CDU leadership.
However, polls show Markus Soeder, the CSU leader, is voters' choice conservative. Some CDU lawmakers want dynamic Health Minister Jens Spahn to run for chancellor, though he has backed Laschet for the party leadership.
The three declared CDU candidates all contrast with Merkel.
Roettgen, 55, the eloquent chairman of parliament's foreign affairs committee, wants Germany to take a firmer stance with Russia and China. Merz, 65, has targeted European Central Bank policy and is less diplomatic. Laschet, 59, who has polished his international profile, complains Berlin has taken "too long to react" to French calls for European Union reform.