Unlike Merkel's party, senior figures in the SPD want to hold off any further debate about the next cabinet's personnel until after their membership vote.
"She speaks the language of the party base and is therefore the right person to pitch the grand coalition", said Carsten Nickel, a Brussels-based analyst at Teneo Intelligence.
Mrs Merkel faces growing internal dissent over her decision to give the post of finance minister to Olaf Scholz, a Social Democrat (SPD), during hard coalition talks last week.
"I am one of those people who sticks to what they promise", Merkel said in a TV interview with broadcaster ZDF on Sunday, cited by Politico.
The turmoil in the SPD can only distract from criticism of Ms Merkel from within her own party after she handed the foreign and finance ministries to the SPD to secure the coalition deal. News weekly Der Spiegel in a cover illustration showed a nude Merkel, with gleeful SPD figures running off with her clothes.
Nahles, a combative, plain-talking former labor minister under Merkel, carries weight in the party base and can hope to shore up support among blue-collar members for another stint in government.
Spahn, fearing a weakening of Berlin's tough austerity-for-reforms stance for indebted eurozone economies, mused that Greek Prime Minister Alexis Tsipras may now "pop the champaign corks".
"The Chancellor has understood", Bouffier told Bild newspaper.
And Ms Merkel herself is in a weak position as she struggles to form a Government after her party lost a number of seats in the recent general election.
Losing the finance ministry was "painful", she conceded, but she stressed that the move was "acceptable", since the alternative would have been a breakdown in talks, and likely snap elections.
That leaves open who from within the SPD may take up that post.
Scholz and Nahles thanked Schulz for his leadership of the party. Both men may now watch the next government from the backbenches. The SPD's 464,000 members still have the chance to veto the decision in a postal ballot. Especially the SPD's left and youth wings fiercely oppose another four years governing in Merkel's shadow.