Anyone hoping for quick approval by Social Democrats was sorely disappointed. Talks between Chancellor Angela Merkel's conservative CDU, its Bavarian sister party, the CSU, and their junior coalition partner, the center-left SPD, over a new deal concerning migrants ended without a definitive result on Tuesday evening.

"We couldn't answer all the questions concerning migration that need to be answered, but we did make some important progress," said SPD Deputy Chairman and German Finance Minister Olaf Scholz, after he, SPD Chairwoman Andrea Nahles and other leaders met with their conservative counterparts.

Scholz said that further discussions within the coalition would take place on Thursday.

On Monday, the CDU and CSU reached a last-minute agreement to head off a potential rebellion by Bavarian conservatives against Merkel. The deal centered on the establishment of so-called transit centers along Germany's border with Austria, which the CSU says would facilitate checks on migrants trying to enter Germany and accelerate deportations.

The CSU is under pressure to demonstrate its conservative credentials on migration to head off a challenge from the far-right Alternative for Germany party in Bavaria's regional election in October. But Bavarian conservatives have some work to do to convince the SPD, which hasn't had much good to say about transit centers in the past. And without the Social Democrats, there's no deal.

A battle the SPD previously won?

From the Social Democratic point of view, one of the biggest problems is that the party already resoundingly rejected transit centers back in 2015 during Germany's previous grand coalition government.

In late 2015, SPD leaders including now-Foreign Minister Heiko Maas, Ex-Foreign Minister and former Party Chairman Sigmar Gabriel and Secretary-General Lars Klingbeil took to social media to portray the zones as "gigantic prisons" and "mass camps in no man's land."

"The SPD has won the day," Gabriel wrote on Twitter back then. "Transit zones are off the table. No house arrest, no fences."

With that in mind, the SPD is almost certain to insist at least upon a change in name for the facilities proposed by the CDU and CSU. Ahead of the Tuesday evening meeting, Nahles told German television that her party "rejects this concept." Nahles also characterized the conservative ideas as a mere sketch, "full of uncovered checks," and not a completely realized plan.

That choice of phrase leaves enough wiggle room for Social Democrats to reach some sort of agreement with conservatives on new migrant processing facilities, provided they're called something else. But whether the SPD's own grass roots will allow the leadership to exploit that leeway is another matter.

Closed camps as a no-go

Early Wednesday morning, the SPD leadership will report on their meeting with conservatives to the Social Democrat parliamentary group. The left wing of the party, meanwhile, has already begun voicing its dissatisfaction with the deal.

"The SPD has clearly said no to closed camps," the influential head of the Social Democrats' youth wing, Kevin Kühnert, told German television. "It doesn't matter whether they're in North Africa, on the external border of Europe or in Passau."

As it is, the SPD is struggling to recover from an internal split over whether it should renew its coalition with Merkel's conservatives, an issue on which roughly a third of its members disagree with the party leadership. Other influential figures including Klingbeil have also said that the Social Democrats will not accept any "closed camps."

Part of the SPD leadership's strategy for winning over their base seems to be to claim that the deal reached between the CDU and the CSU doesn't really change anything. Over the weekend, to little fanfare, the SPD presented its own five-point plan on migration. Social Democrats could well insist that conservatives agree to back their proposal for a general law governing immigration to Germany in return for the SPD backing the CDU-CSU compromise.

It's in the SPD's best interest to facilitate a reconciliation between Germany's two conservative parties. A divorce between the CDU and CSU would almost certainly prompt a fresh national election. That's something that the SPD, currently mired in historic lows of 16 to 18 percent in public opinion polls, can ill afford.

But if there's any comfort in schadenfreude, it's on the way. On Wednesday morning, Merkel addresses the German parliament, the Bundestag, where she's sure to face a scornful grilling from the opposition about the messy spat over migration among conservatives. The SPD can, for a moment at least, simply sit back and enjoy the show.