A clutch of negotiators from her Christian Democratic Union (CDU), its Bavarian allies the CSU and the Social Democrats (SPD) met in Berlin with pressure mounting before a full-table session, which was delayed and expected to run well into the morning hours.
"I'm preparing for a very long night. I've packed a toothbrush," quipped CSU general secretary Alexander Dobrindt, while SPD whip Thomas Oppermann struck an upbeat note: "Today's the finale, I think we'll manage it."
With an earlier loveless marriage from 2005 to 2009 - when the SPD governed in Merkel's shadow, only to be trounced at the ballot box in two elections in a row - SPD chief Sigmar Gabriel faces scepticism among members.
Upping the stakes, he has pledged in an unprecedented move to let the more than 470,000-strong rank-and-file have the final say on any coalition deal in a postal ballot, the outcome of which would be known by around December 14.
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Wrangling continues over how the minimum wage would work, while agreement on another SPD red line issue on easing a ban on dual nationality - a key issue for Germany's large Turkish community - remains elusive.
A handful of compromises have been teased out, such as on rent control and a women's boardroom quota.
Other sticking points in the horse-trading include pensions policy and childcare subsidies as well as ministerial posts.
Media reports say the outlay for both parties' wish list would amount to at least 50 billion euros (USD 68 billion), more than three times what is apparently available.
"If you say 15 billion you're nearer to reality," CDU general secretary Hermann Groehe said.