BERLIN: Germany’s Social Democrats start campaigning on Saturday ahead of a party referendum that spells the last threat to Chancellor Angela Merkel’s hopes of forming a new government, five months after inconclusive elections. In a vote expected to be tight, the more than 460,000 members of the deeply divided center-left SPD will cast their ballots on a plan to enter a new coalition as junior partners to Merkel’s conservatives. The vote, which starts on Tuesday, comes as the 153-year-old labor party’s ratings are in freefall, with latest polls giving it just 16 percent support—only one point ahead of the far-right Alternative for Germany (AfD). If the SPD rank-and-file give the thumbs up in the results to be announced on March 4, veteran leader Merkel will likely launch her fourth-term government by late March. If they vote no, Germans will probably head back to the polls for snap elections, prolonging the political limbo in Europe’s biggest economy and threatening the end of Merkel’s 12-year reign.