The SPD appeared to have struck gold when it nominated Martin Schulz, the former president of the European Parliament, to challenge Angela Merkel for chancellor in the 2017 elections. In what came to be dubbed as the "Schulz effect," the SPD's shake-up at the top saw the party surge in the opinion polls to up to 33 percent, neck-and-neck with Merkel's conservatives.

By May, however, the Schulz effect had faded. If the SPD wanted to prove it was a serious contender for September's federal election, it needed to retain its place at the helm of two key state governments. The first state election in Schleswig-Holstein, however, saw the CDU record an "easy" victory, winning by five points. The CDU went on to form a coalition with the Greens and Free Democrats.

Things went from bad to worse a week later, when Merkel's party defeated the SPD in their stronghold in North Rhine-Westphalia. The SPD had ruled NRW for 46 of the past 51 years. However, it only picked up 31 percent of the vote this time, seven points lower than in 2012. Schulz described the defeat as "crushing," while outgoing state premier Hannelore Kraft resigned as state SPD party chief.

Come September, with the federal election just weeks away, some pundits still gave Schulz half a chance of clinching the chancellery. However, the televised election debate between the two candidates exposed the effects of a grand coaltion. The two leaders seemed happy to agree with the other's proposals, rather than have a debate. And yet viewers still saw Merkel as the more credible candidate.

In its worst election showing since World War II, the SPD scraped just over 20 percent of the vote. It would claim just 40 seats in the Bundestag, 40 fewer than before. Schulz, however, vowed to stay on and lead the party in opposition. The move was widely welcomed by the party's rank and file. It was time for a much-needed recharge and reappraisal.

After talks to form a coalition with the Greens and FDP collapsed in November, Merkel had no choice but to turn to Schulz to form a governing coalition. After much "umming" and "ahing," the SPD leadership decided to enter preliminary talks with the conservatives. Well aware that the move would be unpopular with many in the party, Schulz said the party's 460,000 would get the final say.

However, no sooner had Schulz agreed on a preliminary deal then senior figures in SPD demanded an extensive overhaul. Schulz's failure to prevent a migration cap, establish a "citizens' insurance" scheme and abolish fixed employee contracts raised serious doubts whether party officials would agree to proceed to formal talks.

Ahead of a party conference that would see SPD members vote on the preliminary coalition plan, the movement against another grand coalition began to gather heavy steam. The face of this grassroots revolt was SPD youth wing head Kevin Kühnert. In a series of impassioned speeches, the 28-year-old didn't just win the backing of youngsters, but convinced several senior figures as well.

Despite the heavy backlash, SPD delegates still approved the coalition deal. Of the votes, 362 were in favor of talks compared with 279 against. In a bid to shore up support, then-party leader Schulz said there would be "tough negotiations" with Merkel's conservatives. But that didn't stop critics from accusing Schulz of making concessions to Merkel.

After the announcement of a coalition agreement, media reports claimed that the finance ministry had been given to the SPD, marking what some consider a major victory for the center-left party. The SPD's Olaf Scholz, the popular mayor of Hamburg, was reportedly tapped to head the ministry. If it proves true, it will be the first time in almost nine years that the SPD controls it.

In early February, tensions inside the SPD reached a fever-pitch. Gabriel told German media that Schulz didn't appreciate the job he had done as foreign minister. Days before, Schulz had signaled his desire to lead the foreign ministry. "The only thing left is remorse over how disrespectful we've become with one another in our dealings and how little someone's word still country," Gabriel said.

Former Labor Minister Andrea Nahles is a strong contender to head the center-left party. She made a name for herself as the leader of the SPD's youth branch. However, her impassioned defense of joining a Merkel-led coalition in January was seen as the key to clinching a majority to move forward on formal talks. But she'll have to wait until April 22, when the SPD will choose its next leader.

If anything, the decision to pursue a Merkel-lead coalition has further dragged support for the party. A poll published in February showed the party at 16 percent if elections were held on February 18, a 2-percent drop from the previous poll a month before. Polls now show the SPD neck-and-neck with the far-right AFD.

SPD members – all 463,723 of them – will now vote on whether the party can join a coalition with the CDU and CSU. The members have until March 2 to submit their ballots with results expected shortly after the due date. Until then, Germany waits on the prospect of a new government.