The Christian Democratic Union (CDU) chose the former chief minister of Saarland on Friday to replace Mrs Merkel as party leader.
This comes despite Dr Merkel's demise as party leader came with deteriorating public approval ratings following her decisions during the Europe Migrant Crisis. Major German parties have tended to determine their leaders without a contest, and this is the first open competition for the CDU leadership since 1971.
The outcome is hard to predict, and the race between the two favourites is expected to be close.
Whoever wins will face towering challenges for the party, which is now drawing roughly 30% at the polls, far below the around 40% it enjoyed during Merkel's heyday.
Pointing to her stamina and political nouse, Bouffier pointed out that the Hamburg soccer club had been through 24 trainers and her rival Social Democrats 10 chairpeople in the 18 years Merkel had led the CDU.
Both have prominent backers, though many CDU grandees - including the chancellor - have held off publicly endorsing a candidate.
A third candidate, 38-year-old Health Minister Jens Spahn, is seen as the outsider with little chance of victory. He says the party needs to start "a real change of generations".
While AKK, 56, is viewed as a keeper of the flame and similar to Merkel with an even temper and middle-of-the-road policies, Merz, 63, has become the torchbearer for those seeking a more decisive break with the chancellor.
The choice will be made by 1,001 delegates at a party congress in Hamburg, many of them professional or part-time politicians at federal, regional or local level.
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Merkel has served as chancellor since 2005 and is often called the "de facto leader of Europe", as the most senior leader in the G7 and the longest-serving head of government in the European Union.
"This is democratic order, which reflects a self-confident party", she said.
For years, Merkel's popularity lifted the CDU and its Bavaria-only sister party, the Christian Social Union. In the 2013 election, they won 41.5 per cent of the vote and only just fell short of an outright parliamentary majority.
She now inherits the task of improving the CDU's political fortunes and trying to win back voters from rivals to the right and left, while working with Merkel as chancellor until Germany's next election.
"We need to get rid of what's slowing us down", Kramp-Karrenbauer said "And for many entrepreneurs in this country, that means too few trained professionals, too much bureaucracy and too high costs. And I'm still chancellor", she said. "But while it seems likely that she will broadly attempt to keep the CDU on a centrist line, she should not be mistaken as a one-to-one copy of Merkel", Sparding said "It seems likely that she will move the party slightly to the right, especially on socio-cultural issues".
During her campaign to succeed Merkel as party leader, she repeatedly noted that she was able to blunt an onslaught by the anti-immigrant Alternative for Germany (AfD) party as it picked up seats in state after state.
Waving orange cards with the slogan "Thank you boss", delegates cheered as Merkel, 64, walked onto the stage at the conference centre in Hamburg, her birthplace, to end an era for the party.
"We will all miss her", he said.