You need 354 seats - out of 707 - to form a government (or rather, to elect a chancellor*). SPD/FDP/Greens (Traffic Light) would have 299.
What the FDP has rejected is a Jamaica coalition i.e. CDU/CSU/FDP/Greens. This would have 392 seats and therefore a majority. The FDP's chairman Christian Lindner rejected Jamaica likely out of the marginalization that would come with it; not only would they suffer the voter share squeeze-out that Merkel subjects minority partners to but in addition they'd have to compete with both CSU and Greens over relevant posts and policies. Given they've just reentered parliament* he likely first wants to consolidate the FDP at a sizable voter share, and you can best do that from the opposition bench.
The FDP dropped below the 5% hurdle in 2013, also got slammed out of state parliaments, and only participates in three state governments right now - CDU/FDP coalition in Northrhine-Westfalia, CDU/Green/FDP in Schleswig-Holstein and SPD/Green/FDP in Rhineland-Palatinate. Compared to the Kingmaker role that the FDP used to have up until ten, fifteen years ago this is an extremely weak and vulnerable position, in particular as CDU/Green coalitions as "another option" have become socially acceptable as well. Some crucial elections in 2018 - e.g. Bavaria in autumn - are places where the FDP wants to keep its current share in surveys in order to gain a Kingmaker role against an opposition bench that now includes the AfD.
Current surveys btw - as of today - see the SPD slightly stronger than in the election and the FDP and Greens pretty much switching places with the Greens as strong as AfD (at 12%) and the FDP at 8% as the smallest party (short of CSU) in parliament.
* In the first two rounds. Third election round a traffic light coalition could win with a minority candidate subject to approval from the Federal President.
What the FDP has rejected is a Jamaica coalition i.e. CDU/CSU/FDP/Greens. This would have 392 seats and therefore a majority. The FDP's chairman Christian Lindner rejected Jamaica likely out of the marginalization that would come with it; not only would they suffer the voter share squeeze-out that Merkel subjects minority partners to but in addition they'd have to compete with both CSU and Greens over relevant posts and policies. Given they've just reentered parliament* he likely first wants to consolidate the FDP at a sizable voter share, and you can best do that from the opposition bench.
The FDP dropped below the 5% hurdle in 2013, also got slammed out of state parliaments, and only participates in three state governments right now - CDU/FDP coalition in Northrhine-Westfalia, CDU/Green/FDP in Schleswig-Holstein and SPD/Green/FDP in Rhineland-Palatinate. Compared to the Kingmaker role that the FDP used to have up until ten, fifteen years ago this is an extremely weak and vulnerable position, in particular as CDU/Green coalitions as "another option" have become socially acceptable as well. Some crucial elections in 2018 - e.g. Bavaria in autumn - are places where the FDP wants to keep its current share in surveys in order to gain a Kingmaker role against an opposition bench that now includes the AfD.
Current surveys btw - as of today - see the SPD slightly stronger than in the election and the FDP and Greens pretty much switching places with the Greens as strong as AfD (at 12%) and the FDP at 8% as the smallest party (short of CSU) in parliament.
* In the first two rounds. Third election round a traffic light coalition could win with a minority candidate subject to approval from the Federal President.
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