Is it a smart strategy of the SPD choosing a member of Merkel's administration?

[Astatine]

Of course, had it chosen to form a Red-Red-Green coalition in 2005 or 2013, this could have invited a backlash if the government was seen as too extreme, and  there is no guarantee that its relationship with die Linke would have been stable and productive, given that it would have been a 180-degree U-turn from the Schröder years. And that's IF they managed to agree to a government deal at all in the first place.

Perhaps if it had emerged as the largest party in 2005 and the CDU made the junior partner, the roles may have been reversed. Who knows.

To be honest my argument in favour of a SPD-Linke-Grune coalition in 2005 (or 2013 but that one is harder and arguably undemocratic but still doable) is basically what happened here.

There are lots and lots of clips of Pedro Sanchez arguing "I will not do a coalition with UP"; "I will not make talks with Bildu"; "There will be no deals with the Catalan separatists", etc.

What did he do? He did a coalition with UP and did talks with Bildu and the Catalan separatists.

Of course the opposition uses those as attacks, yet the attacks don't really seem to stick all that much.

Hindsight of course, but Pedro Sanchez learned the lesson from the German SPD and the Greek PASOK; and he learned it the hard way after the 2016 Spanish election. A coalition with the far left is always going to be the lesser evil electorally. A coalition with them might hurt your polling numbers slightly. A coalition with the centre-right will sink your party forever.

Current Spanish polling does reveal that those decisions were unpopular, even among PSOE's electorate; yet the party is still polling decently well and has completely defused the possibility of UP beating PSOE. Meanwhile the fact that Grune will beat SPD is essencially a certainty now.

A coalition in 2005 wouldn't have worked for various reasons:

- The SPD's surprisingly good result can mostly be attributed to Gerhard Schröder's personal popularity. While the majority clearly disapproved of the red-green governmental performance, Schröder himself was still far more popular than Angela Merkel. The thing is... The Left Party (or alliance, as PDS and WASG had not merged yet) in that form was basically founded by Schröder's biggest rival, Oskar Lafontaine, who abruptly resigned from his position as Finance Minister in 1999 due to disagreements with Schröder. As far as I know, they haven't talked in years. So either Schröder - who almost won the election against all expectations - would have to go or the Left would have fallen apart before it was even formed as party.
Especially on foreign policy, parts of the Left are still considered unreliable (especially by Greens and centrist SPD members).

- A coalition with the Left/PDS was not as acceptable at that time as it is nowadays. The PDS was still widely seen as the successor of the DDR state party SED, and the CDU regularly did negative campaigning against a constellation in which SPD and PDS would cooperate ("Rote-Socken-Kampagne"/red sock campaign). In 1994, the SPD in Saxony-Anhalt ousted the CDU from the state government by forming a minority government tolerated by the PDS, and that might have been a factor that cost SPD/Greens a win in the federal elections later that year. Opening up towards the PDS/Left Party was a long process, and I believe that the country was simply "not there yet" in 2005.

With hindsight, calling the snap elections at that point might have been Schröder's dumbest move. If he had waited until the regular end of his term, the SPD would have either lost quite badly, but could recover as major opposition force (if a Union/FDP coalition had imposed the radical reform programs they intended to realize, there would have likely been a strong backlash) or would have become largest party based on the first signs of economic recovery.

[Astatine]

On these notes, can someone tell me why Linke is doing so poorly while it is Grüne the one that is gaining from the SPD collapse?

If you are one of the people abandoning the "SPD sellouts who always go with CDU" it makes no sense to vote Grüne, who will just as easily do coalitions with CDU?*

Is Linke still suffering from DDR stigma? Fear of Linke being a pariah party like AfD? Or is it something else?

*: Or actually even more so, one of my impressions from green parties in Europe is that they tend to "sell out" even easier than the socdem parties

Because the Greens and die Linke are completely different parties. Die Linke is unambiguously a left-wing, socialist party, whereas it is doubtful whether the Greens are even to the left of the SPD any more; they are essentially a moderate social liberal party. Their voter bases are completely different; the most fundamental difference is that die Linke are an East-based party, whereas the Greens’ strongholds are in the West, and more broadly the Greens’ coalition is significantly more upscale and educated. Whereas die Linke are probably competing with the AfD for a good amount of votes, the Greens have been able to pick off upper middle class professionals from the CDU. I would say that the Greens now have the bulk of younger centre-left voters, and it is mostly older people sticking with the SPD.

Die Linke has been changing though for the last decade. The party and its electoral base are becoming younger, more Western and more educated. The problem is that the new base of very progressive voters is growing but limited and that competition by smaller parties like DIE PARTEI, Pirates, Volt, Democracy in Movement, Animal Welfare, etc. can lead to electoral desasters like the last European Elections.

Created this little map to visualize the performance of the Left (respectively PDS/WASG alliance) in 2005 and 2017. The national result (8.7 and 9.2 % respectively) was almost identical, but you can clearly see how the Left lost ground in the East (+Saarland with the vanishing Lafontaine factor) while simultaneously gaining support in the West.