The embattled Chancellor’s fourth term hangs in the balance after failing to form a government four months after Germany’s general election.
She announced last Friday that she had finally reached an agreement with her old coalition partners, the SDP.
Now, the party leaders must sell the deal to party members before a congress later this month at which delegates vote on the agreement.
But the SDP leader in the German parliament, Andrea Nahles, said there is “no plan B” should German coalition talks fail.
Ms Nahles told German newspaper Berliner Morgenpost, said: “Many are also coming out in favour of formal coalition talks.
“The SPD is just a very lively party, struggling to find the best way forward together.”
“We’re not a party that will simply rubber-stamp something, but neither will we walk away.
“I am optimistic. We didn’t push as hard as we could have in these talks.
“Many core SPD issues for which we’ve been fighting for years are now possible.
“We’ve discussed the big issues … Negotiations are exhausted at certain points.”
Ms Nahles insisted holding a new election is not an option.
There is still a way to go before the new coalition can be confirmed and leading movements of the SDP have vowed to press for improvements to the blueprint agreed at the end of last week.
SDP party delegates will vote on Sunday on whether or not to progress to formal talks with Ms Merkel’s Christian Democrats (CDU) and their Bavarian sister party, the CSU, based on the position paper drawn up during exploratory talks.
If the SPD votes against the deal it will spell disaster for leader Martin Schulz, who will almost certainly be expected to resign after rowing back on a campaign pledge not to enter a coalition government with the CDU.