The 60-year-old leads a coalition of the SPD and the Christian Democrats in his northern state. In the Bundesrat — the council of Germany's 16 states and upper house of parliament — he serves on the Committee on Foreign Affairs and is deputy chairman of the Committee on European Affairs. Many in the SPD have long seen him as a possible new hope to lead the party forward.

Schwesig, born and raised in the former East Germany, is the first woman to lead the state of Mecklenburg-Western Pomerania. The 45-year-old previously served as minister of family affairs under Chancellor Angela Merkel, a role she used to push for equal opportunities for women and better state childcare facilities. She serves on the Bundesrat's Committee on Foreign Affairs.

Scholz, 60, is seen by some of his fellow party members as a technocrat and is considered to belong to the SPD's conservative wing. As finance minister under Merkel, he has been committed to curbing public spending and keeping a lid on new debt. He also served as federal minister of labor and social affairs in Merkel's Cabinet from 2007 to 2009.

A reasonably popular figure in the SPD, Dreyer was voted in as deputy party chairwoman in 2017 with 97.5% backing. Despite repeatedly stating she would not want to leave her western state for Berlin, she is still seen as a possible replacement for Nahles. The 58-year-old has served as premier of Rhineland-Palatinate since 2013 and is the first woman to hold the office.

The 59-year-old Bundestag member from Cologne is one of the deputy chairs of the SPD's parliamentary group. In that role, he is responsible for foreign affairs, defense and human rights policy. He is well respected within the party and could be tapped as interim SPD leader in the Bundestag.

The 60-year-old sociologist leads the SPD in Germany's most populous state, North Rhine-Westphalia. He is a deputy chair of the SPD's parliamentary group, overseeing budgetary affairs, finance and Europe. As secretary-general of the Party of European Socialists, he is also considered well connected at the European level.

The former president of the European Parliament has been suggested as another possible challenger. The 63-year-old ran as the SPD's candidate for chancellor in 2017 but lost to Merkel. He subsequently stepped down as SPD leader. He has already said he doesn't want the job back, but Germany's Bild am Sonntag newspaper reported last week that he was keen to lead the SPD parliamentary group.

The first democratic government to rule West Germany since the end of World War II saw Christian Democratic Union leader Konrad Adenauer form a governing coalition with the Free Democrats and the German Party (a now-defunct national conservative party). When Adenauer's conservatives won re-election four years later, he once again turned to the same coalition partners.

After four years of ruling West Germany on their own between 1957 and 1961, the conservative Union lost their majority in the Bundestag and were forced to enter into coalition with the Free Democrats again. Adenauer resigned in 1963 for his part in the so-called "Spiegel" scandal. His Minister of Economic Affairs Ludwig Erhard (left) was elected by parliament to take over

The first ever "grand coalition" was not the product of an election. Ludwig Erhart was re-elected in 1965 and continued to rule alongside the FDP. However, the following year the Free Democrats left the government over budget disputes. Erhart also resigned and Kurt Kiesinger (right) was chosen to take over. With the FDP out, he governed with the Social Democrats, led by Willy Brandt (left).

Willy Brandt became Germany's first Social Democratic chancellor in the post-war period. Despite winning fewer votes than the CDU/CSU, Brandt struck a deal with the FDP to give them a narrow majority in the Bundestag. It wouldn't be the last time the liberals would be called out for a perceived lack of loyalty. In 1974, Brandt was replaced by Helmut Schimdt, who went on to win two more elections.

The 13-year friendship between the SPD and FDP ended in 1980 as the two parties' differing ideologies became irreconcilable. The liberals again switched sides that year, dropping out of the coalition and seeking a deal with the conservatives. That caused the SPD-led government to collapse and a reborn CDU/CSU-FDP coalition formed under the leadership of Helmut Kohl (pictured).

Shortly after the fall of the Berlin Wall, East Germany held its first ever elections. The Christian Democrats under Lothar de Maiziere took over 40 percent of the vote. They went into coalition with two small parties: German Social Union and Democratic Awakening, whose members included one Angela Merkel. In October that year, the government signed the reunification treaty with West Germany.

In 2002, Helmut Kohl's 16-year rule came to an end and the Social Democrats under Gerhard Schröder returned to power. The SPD formed a coalition with the Green party, who became a governing party less than 20 years after being founded. Unlike under Brandt, the SPD now led a left-wing government, rather than a center-left coalition. The SPD-Green party coalition remained in power until 2005.

"Grand coalitions" do not come easily. When the first exit polls came in, both Schröder (right) and Angela Merkel (left) declared themselves the winner. In the end, Merkel's conservatives defeated the SPD by just 1 percent. Germany's two largest parties agreed to form the country's second-ever grand coalition.

The "grand coalition" experiment ended in 2009, after the SPD picked up a disappointing 23 percent in the federal elections. The Free Democrats, by contrast, gained almost 5 percent to give them over 14 percent of the vote. Merkel and the FDP's Guido Westerwelle (left) formed a coalition with relative ease. It was, after all, Germany's 11th CDU/CSU-FDP government.

After taking more than 40 percent of the vote, Merkel's conservatives probably weren't expecting to rule with the SPD. But with her old allies the FDP failing to meet the 5 percent threshold to enter the Bundestag, options were limited. Merkel called on the SPD to join her and "take on the responsibility to build a stable government." She made the same speech again four years later.