The current fragmentation of the political left in Germany could barely be better symbolized than by what happened this year again at the annual silent remembrance in Berlin at the graves of Rosa Luxemburg and Karl Liebknecht, the two communists murdered by radical right-wing paramilitaries in January 1919. On the morning of January 14, as they always do, leaders from the Left Party turned out, while leaders of the Social Democratic Party (SPD), as is also customary, stayed away.

Not only is there a rift between these two leftist parties; the parties themselves are divided within themselves. Social Democrats are split over whether to form another centrist "grand coalition" with Angela Merkel's conservatives, with the local rank and file noticeably less enthusiastic about the idea than the party's national leadership.

Meanwhile, the Left Party's two best-known and most charismatic leaders, Sahra Wagenknecht and Oskar Lafontaine, openly speculated over the weekend about forming a new big-tent movement to replace both the SPD and their own party. Predictably, other Left Party leaders weren't all that keen on that idea.

Leftist politicians' inability to agree on policies or leaders is turning voters off. The SPD barely cracked 20 percent in last year's election – its worst-ever national result. And at present, there is no clear way forward, either in terms of policy or personnel, to reverse a long-term fragmentation.

2005: the year the German left split apart

Arguably, the splintering of the German left began with the rise of the Green party in the early 1980s, but it became undeniable with the establishment of the Left Party in the mid-00s. The crux came in 2005 when Chancellor Gerhard Schröder engineered an early national election as a referendum on his Agenda 2010 reforms, which significantly cut Germany's social welfare system in the interest of growth and economic competitiveness.

Much like Merkel today, Schröder was a nonideological moderate, but the electorate didn't reward his centrism. More than 2 million voters deserted the SPD, and Schröder was forced to cede the chancellorship to his conservative rival.

Disunity on the left was a decisive reason for Schröder's narrow defeat – the conservative CDU/CSU got only 1 percent more of the vote than the SPD. Exploiting left-wing disgruntlement with the Agenda, the Party of Democratic Socialism (PDS) – the direct successor to the state socialist party in former communist East Germany – more than doubled their share of the vote in 2005, taking 8.7 percent.

The PDS, with its strongholds in the formerly communist East, subsequently merged with the WASG, a splinter group of western German leftists unhappy with the centrism of the SPD, to form the Left Party. Implicit in the name change was a claim to represent all those who felt that the SPD wasn't left-wing enough. Its appeal has remained remarkably stable, with the party taking between 8.6 and 11.9 percent of the vote in the previous three national elections.

Meanwhile, the SPD has gone from 34.2 percent of the vote in 2005 to only 20.5 in 2017. The lion's share of that vote loss is the approximately 10 percent of the electorate who under other circumstances would probably be voting SPD but now support its rivals on the left. Left Party supporters now have a decade of history voting for that party and are unlikely to go back to the Social Democratic fold – especially as Social Democrats are caught in a vicious circle.

Read more: The SPD: Germany's oldest political party

The east-west divide, 'grand coalitions' and the splintering of the left

Schröder's inability in 2005 to keep the left wing of the SPD's potential support base loyal didn't just cost him power. It also made the mathematics of parliamentary majorities in Germany much more difficult. As a result, Social Democrats have been perennially pressured to serve as junior partners in a coalition with their historical political adversaries, the conservatives, thus further alienating the left wing.

The weakening of the left in the Merkel era hasn't brought with it lasting gains on the political right. On the contrary, that era began in 2005 with more than 50 percent of voters casting ballots for one of the left-wing parties – SPD, PDS or Greens. The conservatives' success has been built on their relative cohesion. By contrast, because of the connection with communist East Germany, no SPD leader since 2005 has dared risk cooperating with the PDS/Left Party.

At the same time, the PDS/Left has eaten up so much of the vote that, in three out of the last four elections, Merkel's conservatives have ultimately had no other option but to seek a centrist coalition. But the big losers in the equation have been the Social Democrats. Support for the SPD nosedived again in the post-"grand-coalition" national election in 2009, and the same trend was repeated in 2017.

This is why many in the SPD rank and file were already complaining about last week's provisional coalition agreement with the conservatives before the ink had even dried on the paper. There simply aren't enough gains to be made in the center, where Merkel's conservatives have locked down most of the vote, to compensate for further losses on the left. It's hardly surprising that Lafontaine, a former SPD chairman from the pre-Schröder era, thinks there's both a chance and a need to blow up the party landscape and unite the left-wing spectrum.

But getting everyone into such a big tent is much easier said than done.

Disturbing parallels

It would be interesting to know whether any of those who paid tribute to Luxemburg and Liebknecht on Sunday reflected on the differences and similarities between then and now.

Long gone are the days when the left was neatly aligned with the industrial working classes. The big-tent leftist movement so vaguely formulated by Lafontaine and Wagenknecht would have to bring together the interests of, for instance, blue-collar workers, the long-term unemployed, affluent environmentalists, human- and civil-rights activists, single parents and Germans from vastly different cultural backgrounds in the east and west.

That's no mean task, particularly as the political left after Schröder hasn't produced any leaders with his charisma. In today's political climate, it's difficult to imagine a leftist movement without a populist identification figure for supporters to rally round. But there are no Luxemburgs or Liebknechts on the horizon.

In one disturbing sense, however, there are parallels between then and now. The left was similarly fractured in the Weimar Republic, split between the SPD, the splinter Independent Social Democratic Party (USPD) and the German Communist Party. Back then, too, the SPD agreed to a number of centrist coalitions and ultimately saw its support decline to just above 20 percent.

The Weimar Republic, of course, ended tragically with a fragmented left unable to respond concertedly to the rise of a populist party on the far right. Political disintegration on the left is arguably bad for the nation as a whole, and left-wing politicians today have indeed recognized the problem. On its main website, the SPD has launched a prominent campaign inviting users to help "renew the party."

The question is whether Social Democrats can attract sufficient numbers of people to take up this offer – and whether anyone will emerge to lead them if they do.