Merz, who wants to shift the center-right party further to the right, is throwing his hat into the ring after Merkel's chosen successor, CDU leader Annegret Kramp-Karrenbauer, gave up her leadership ambitions on Monday in a deepening party crisis over ties between the centre and far right.
That is also why the decision by Ms. Kramp-Karrenbauer, 57, to step aside raised uncomfortable questions about the direction the Christian Democrats will take after Ms. Merkel, 65, leaves power next year.
She believes the same person should serve as both chancellor and party leader and will organise a process in the summer to fill both roles, the source added.
In a party press conference on Monday, Kramp-Karrenbauer said splitting the posts of chancellor candidate and CDU leader, a move backed by Merkel, had been a mistake.
Addressing reporters in Berlin, Kramp-Karrenbauer said the AfD stood "against everything we as the CDU represent", adding that "any convergence with AfD weakens the CDU". She will stay on as Germany's defence minister. "I have invited those whose names are now circulating for one-on-one interviews", Kramp-Karrenbauer said in a television interview.
Laschet, who as premier of North Rhine-Westfalia, a state of 17 million people, has executive experience that other candidates lack, was more forthcoming than in the past.
AKK is also more conservative than Merkel, particularly on social issues such as gay marriage, which has tempered Merkel's willingness to support her.
The next CDU leader will also be the party's candidate to become Chancellor at elections due by the end of 2021.
The CDU has struggled to gain support in state elections, where parties on the right and left have swayed voters.
The Alternative for Germany welcomed Kramp-Karrenbauer's resignation, as did Germany's former domestic intelligence head, Hans-Georg Maassen, a vocal figure on the right of Merkel's party since his ouster as Germany's spy chief in 2018. Its success has complicated Germany's political tradition of governing with multi-party coalitions, as most of its rivals have ruled out working with Alternative for Germany. Despite her orders to work with neither the far-left (Die Linke) nor the AfD, Thuringia's 22 CDU parliamentarians voted with the AfD anyway.
"Whoever becomes the new chairman of the CDU needs to ensure that the party remains a reliable partner", Heiko Maas, a member of the Social Democrats, tweeted. "But one lesson from the Weimar Republic is that far-right extremists come to power because mainstream conservatives want to use them to remain in power", Der Spiegel staff wrote.
Last week, Kramp-Karrenbauer's inability to impose discipline on the conservative Christian Democrats in the eastern state of Thuringia dealt a fresh blow to her credibility - already eroded by a series of gaffes.