Germany's conservative Christian Democrats are set to vote on Friday in a close-run race to succeed Chancellor Angela Merkel as CDU party chief with three main contenders having emerged for the powerful post.

While polls of party members show Merkel ally Annegret Kramp-Karrenbauer as edging ahead in the battle for the CDU's top job, a former CDU heavyweight-turned-businessman Friedrich Merz has secured the backing of several key senior party figures in his bid to take over from Merkel.

Health Minister Jens Spahn is also in the running, but not considered a serious contender for the top job.

Securing the CDU party head when 1,001 delegates vote on Friday at a party conference in Hamburg would represent a major step by the winner towards becoming chancellor once Merkel bows out of power in 2021.   

The 56-year-old Kramp-Karrenbauer, a former state premier who has been dubbed "mini Merkel" because of her pragmatic centrist political style, was elected CDU general secretary in February as part of efforts to give the CDU fresh momentum.

In addition to sections of business, the 63-year-old Merz has also won support from conservative CDU members who see him as pushing the party to the right, thereby standing up to the challenge represented by the anti-foreigner Alternative for Germany (AfD) party.

Merz's bid to succeed Merkel comes nearly 10 years after he left the national political stage when he lost out to Merkel in a party power struggle.