Seehofer, whose party is more socially conservative than Merkel's, has always been a critic of the chancellor's immigration policies, particularly her decision to open Germany's borders at the height of the refugee crisis in 2015.
German Chancellor Angela Merkel, who Trump met with last week at the G7 summit, has faced political headwinds in the country since she implemented an open-door policy in 2015 in an effort to lead the way in response to the refugee crisis. The AfD that swept into Parliament a year ago on a wave of anti-migrant sentiment is staging a march Sunday through the heart of Berlin to protest against the government.
Merkel insisted Monday that the CSU and CDU have the same goal - to better control immigration - but that it was an issue that can only be resolved on the European level.
In an unprecedented challenge to her 13 years as Germany's chancellor, a resolution by her interior minister, Horst Seehofer, is set to be passed on Monday, party aides told German newspaper Bild Zeitung.
Seehofer wants Germany to have the right to reject migrants who have already registered in another European Union state but Merkel opposes any unilateral move by him that would reverse her 2015 open-door policy and undermine her authority.
Merkel is locked in a standoff with her interior minister, Horst Seehofer, over his demand for some asylum-seekers to be turned back at the border.
Berlin is also reportedly preparing to call a meeting between Merkel and the leaders of several European Union frontline nations in the migrant crisis ahead of the European Union summit.More news: 23-Foot-Long Python Devours Indonesian Woman
Both party leaders are expected to address the media on Monday afternoon.
A CSU leadership meeting Monday in Munich is likely to authorise Seehofer to go ahead with his plan - but it's unclear at what point leaders want it to take effect.
The arrival of more than a million asylum seekers to Germany since 2015 has deeply divided the country and propelled the far-right AfD into parliament for the first time in September's general election.
Several high profile crimes by migrants have also fuelled anger. They include a deadly 2016 Christmas market attack by a failed Tunisian asylum seeker and the rape-murder in May of a teenage girl, allegedly by an Iraqi. The CSU is determined to show that it's tough on migration, arguing that this is the best way to cut support for the anti-migration, far-right Alternative for Germany party ahead of a challenging state election in Bavaria in October. "We want a national solution if a European one is not found", Seehofer said, adding that "the CDU or at least its leaders do not want" it. Seehofer has said these migrants should be turned away at the German border whereas Merkel has said this can only happen with the agreement of the relevant European Union states.
"The situation is serious but still solvable", he wrote.
Alternatively, if the CSU splits from the CDU and becomes an opposition party, the governing coalition would be left with 353 seats, one short of a majority, potentially triggering the collapse of the government.
Seehofer has earned the wrath of politicians across the political spectrum for appearing to seek a major confrontation with Merkel, threatening to place police at the border even if she rejected his plan, which would leave her little choice but to sack him.