For years, Merkel's popularity lifted the CDU and its Bavaria-only sister party, the Christian Social Union.
HAMBURG-Chancellor Angela Merkel's party celebrated her 18 years as its leader with a lengthy standing ovation on Friday as it prepared to elect a successor who could help shape Germany's political direction for the next generation.
Kramp-Karrenbauer said she wants to ensure that the CDU avoids the fate of shrinking center-right parties in France and elsewhere.
She has avoided debates about Mrs Merkel's decision to accept large numbers of refugees into Germany in 2015, but at other times has advocated a tough anti-migrant stance, proposing a life ban on entry to Europe for asylum seekers convicted of serious crimes.
"Whether it's the rejection of multilateralism, the return to nationalism, the reduction of global cooperation to deal-making or threatened trade wars. hybrid warfare, destablisation of societies with fake news or the future of our European Union - we Christian Democrats must show in the face of all these challenges what we've got", she said.
Annegret Kramp-Karrenbauer, a former state governor and now the general secretary of the CDU, is regarded as one of the favourites to succeed Merkel.
Health Minister Jens Spahn, another Merkel critic, is considered the outsider.
The chancellor had announced that she planned to step down this month, adding that she will not run again as Chancellor in the next general elections in 2021.
"If they want more of what Merkel stood for then they will go for Annegret Kramp-Karrenbauer but I don't think that would be the best option, frankly".
Sometimes dubbed "mini Merkel", Kramp-Karrenbauer is admired by the CDU upper echelons for her appeal across the party. More generous family leave, an exit from nuclear power and an end to military conscription were among her signature policies.
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In his speech, Merz highlighted the need to tackle the "intolerable" success of the far-right Alternative for Germany party and called for a more combative approach toward the CDU's rivals in the political center ground.
That isn't automatic, but all but two of Kramp-Karrenbauer's seven predecessors as CDU leader became chancellor.
"Even if among voters there seems to be a clear preference for AKK, it doesn't look that clear-cut among CDU delegates who are the ones voting at the end of the day", said Famke Krumbmueller, a Paris-based partner at political-risk consultancy OpenCitiz.
Above all, Merz had underscored his promise to move for a clean break from Merkel's 13 years in power.
She joined the CDU in 1981, first serving in her local youth division of the party.
"Schaeuble's manoeuvre shows: the CDU of the old Germany is trying to make a comeback", news weekly Der Spiegel said. "Opposing camps are forming".
One of the most recent examples of Merz wanting a change of course for the CDU has been his outspoken criticism of the United Nations migration pact, which was revealed to have been largely drafted by the Merkel government starting in 2016. Whoever comes out on top will be seen as chancellor-in-waiting - a status conferred on CDU leaders since the foundation of the federal republic after World War II.
One senior CDU official, speaking on condition of anonymity, said many delegates were undecided before the Congress and could be swayed by how the candidates present themselves on Friday.
"We can't afford another year like 2018 when we fought so much", he told public broadcaster ZDF.