Merkel will meet with Social Democrat leader Martin Schulz and the chief of her Bavarian allies, Horst Seehofer, at 0800 GMT, sources close to the talks told AFP.
As Germany's European partners are beginning to weary over the long-running impasse along with many Germans, Schulz said the negotiations would proceed "full speed ahead... without getting carried away at all."
He said the SPD was broadly optimistic and determined to forge an alliance that the party leadership could present to the rank-and-file "in good conscience".
But the outcome is still uncertain as the SPD will give its 440,000 members a vote on the end result.
September's election left Merkel without a majority and struggling to find partners to govern, as the far-right AfD party capitalised on anger over a record influx of asylum seekers to snatch voters from the established parties.
After Merkel's bid to form a government with the smaller left-leaning Greens and pro-business FDP fell through, she was forced to woo back the SPD, its partner in the last four- year-term.