A significant minority of Merkel's conservatives may vote against the 86 billion-euro ($94.84 billion) bailout, sending the government a warning that the latest package is its last chance to keep debt-ridden Greece in the 19-country euro zone.
In a test ballot of conservative lawmakers late on August 18 - a non-binding vote - a clear majority voted in favor of the bailout. Sixty of the 311 conservative lawmakers voted `no' or abstained - fewer than a rebellion by as many as 120 MPs that had been feared.
The vote on August 19 in parliament will follow a debate scheduled to start at 09:00 a.m. (0700 GMT).
Merkel is not expected to address the lawmakers. She will leave her highly respected finance minister, Wolfgang Schaeuble, to speak for her government.
Last month, he argued that Athens should consider a "timeout" from the euro zone, before later telling parliament that talks on the third bailout were a "last attempt" to solve the crisis.
Schaeuble threw his weight behind the package before August 19's vote and said Athens was ready to reform. Senior figures in the ruling coalition agreed.
"One has to recognize that the Greek government has in recent weeks made a lot of concessions that it rejected for months," Gunther Krichbaum, conservative chairman of parliament's Europe committee, told Reuters.
Last month, a record 65 lawmakers from the conservative camp broke ranks and refused to back negotiations on the bailout. The newspaper Bild estimated as many as 120 CDU and CSU members out of 311 might refuse to back the now-agreed deal.
Support from parties including the Social Democrats, Merkel's junior coalition partner, and the opposition Greens means approval of the bailout is not in doubt. But a rebellion by a large number of her allies would be a blow for Merkel, who remains highly popular after 10 years in office.
On August 18, the parliaments of Austria, Estonia and Spain voted to approve the bailout.