The Social Democrats (SPD), having only just emerged from a tussle at the top of the party that saw its chairman Franz Muentefering resign, huddled in Potsdam, just outside Berlin, in an appeal for unity ahead of the final coalition talks.
The Christian Democrats were to join Merkel at a meeting later in Berlin and define their room for maneuver on a range of economic and foreign policy issues before the self-imposed November 12 deadline to form a "grand coalition" government.
Merkel, who is to become Germany's first woman chancellor, told supporters Saturday that she was confident the two parties would succeed in forging their power-sharing alliance this week with a clear government program.
"We - and that means all sides involved - will make a concerted effort to form a grand coalition... that addresses the real problems of the country and leads to solutions," she said.
Muentefering, who made way this week for popular east German state leader Matthias Platzeck as SPD chairman after a leftist rebellion, said the notoriously fractious party it must pull itself together in the coming days.
"The carelessness with which we talk about each other and to the media is destroying us," Muentefering told an SPD meeting in the northern state of Lower Saxony, referring to vicious battles between the party's left and right wings.
Despite stepping down as party leader, Muentefering is to serve as Merkel's vice-chancellor and labour minister.
He, Platzeck and outgoing Chancellor Gerhard Schroeder sent a joint letter to the party saying it had a responsibility to stop the infighting, particularly at a crucial time for the future government.
Merkel faced turmoil of her own this week when a key ally and sometime rival, Bavarian state premier Edmund Stoiber, suddenly announced he would not join her government as planned as economy minister, citing the "unreliability" of the SPD.
She is also facing public grumbling by powerful conservatives that the compromises being hammered out with the Social Democrats will do little to kick-start the moribund economy and drive down the 11-percent jobless rate.