Spahn, a champion of the right in Merkel's Christian Democrats (CDU) and vocal critic of the chancellor, told Deutschlandfunk radio: "The SPD must decide: either we rule together or some will try to play opposition within the government".
Chancellor Angela Merkel said on Monday she would work with France to tackle pressing issues such as trade policy, the war in Syria and competition with China after the Social Democrats (SPD) approved joining a coalition with her conservatives.
Both Merkel's conservatives and the centre-left SPD had been weakened as voters angry about the arrival of more than a million asylum seekers in Germany since 2015 turned to the far-right Alternative for Germany (AfD).
Both Merkel's conservatives and the SPD are under pressure to appear distinctive to voters in a coalition borne out of necessity rather than choice, making it hard for Merkel to balance conflicting demands. Though no chancellor has given up leadership voluntarily, without an electoral defeat or a major scandal, none has served longer than Helmut Kohl's 16 years and 26 days, and that makes it likely that Merkel, visibly exhausted by her never-ending battles, won't vie for a fifth term in office in 2021.
The idea was to keep one of the most influential leaders, Merkel as the Chancellor in Berlin. Her party received the most votes but it took five months to form a coalition government.
Ralf Stegner, current leader of the SPD in Schleswig-Holstein, told local TV Phoenix that the vote is a good government basis, and the 33 plus percent who voted against the Grand Coalition would be a reminder.
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Later in the speech she did speak about the use of technology at the border and a customs arrangement (but not a customs union). But she sought to reassure companies that an alternative system would be set up.
But the two blocs' weak performances in last September's election, when they both suffered their worst results since Germany became a republic in 1949, mean the latest incarnation of their awkward alliance promises to be the most fractious yet.
Two in three of the SPD's rank and file voting in a crunch referendum backed a new partnership with Merkel's conservatives, heralding an end to the political stalemate that has plagued Europe's biggest economy since September's inconclusive elections.
"We now have clarity: the SPD will join the next German government", he added. Immigration is expected to be a key issue in this fall's state election in Bavaria, where Merkel's allies fear the fallout from her open-door policies. It also came after Martin Schulz was forced to hand over the reins as chief of the Social Democrat Party.
According to the coalition agreement, the SPD administration must announce the ministers' names by March 12. The radical youth wing had opposed the coalition.
All that "requires us to begin work quickly in the government", she said.
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