Merkel's center-right CDU party and its conservative Bavarian CSU allies are holding separate meetings to weigh the results of last week's EU summit, which agreed collective measures by the bloc's 28 members to reduce immigration.
Merkel hopes the deals with Germany's neighbors will deter Interior Minister and CSU leader Horst Seehofer from defying her by turning away at the border asylum seekers already registered in other EU nations. The unilateral move would force her to fire him, in turn prompting a CSU walkout that would cost her her majority in parliament.
According to a document sent to coalition partners, Merkel sought to assauge the hardliners with deals with 16 other countries to return already-registered migrants if they reached Germany.
The German leader, who has been in office since 2005, has warned that the issue of migration could decide the very future of the EU itself.
The EU and bilateral deals were "only possible because the chancellor enjoys respect and authority throughout Europe," Germany's EU Commissioner and CDU politician Guenther Oettinger said in an interview with the Frankfurter Allgemeine Sonntagszeitung weekly.
But several central European nations including Poland, Hungary, the Czech Republic and Slovakia denied they had agreed to accept returned migrants.
"Given the different statements from some EU member countries, one can doubt whether all of the decisions at the EU Council will become reality," head of the CSU parliamentary group Alexander Dobrindt told Bild am Sonntag newspaper.
Leaders and MPs from the Bavarian party will meet in Munich to decide their response, while Merkel and her top lieutenants will gather in Berlin.
The chancellor's frantic last-minute diplomacy was ultimately prompted by the CSU's fear of losing its cherished absolute majority in Bavaria's state parliament.
The "Free State" with its beer-and-lederhosen Alpine traditions, powerful industries and impenetrable dialect has a more conservative bent than other German regions.
Newspaper headline: Merkel defends migrant deal with EU