BERLIN (Reuters) - Germany’s Social Democrats (SPD) are expected to choose their long-standing chairman Sigmar Gabriel to run against conservative Angela Merkel for chancellor in September’s federal election, senior party sources said on Thursday.

German Economy Minister Sigmar Gabriel addresses a news conference in Berlin Germany, December 19, 2016. REUTERS/Fabrizio Bensch

Gabriel, vice chancellor and economy minister in Merkel’s right-left coalition for the last four years, is more popular with the SPD rank and file than his recent predecessors but also has a reputation for being impetuous and unpredictable.

“There’s no way around Sigmar Gabriel as candidate for chancellor at this point,” one senior party source told Reuters.

“The likelihood is very, very, very high that Gabriel will be the candidate for chancellor,” a second source and member of the party’s executive committee added.

Party officials stressed, however, that no final decision was expected until the end of January.

Some party members had hoped that Martin Schulz, the former president of the European Parliament, would be named as the SPD’s top candidate in the election, with polls showing he would get more votes than Gabriel in a matchup with Merkel.

That looks unlikely, especially now that the party is expected to name Schulz as foreign minister to replace Frank-Walter Steinmeier, whom the coalition parties agreed to elect as German president on Feb. 12.

Gabriel, a 57-year old former school teacher, trails Merkel in popularity ratings.

A new poll released on Thursday by German broadcaster ARD and Infratest Dimap showed the SPD winning 20 percent of the vote, compared to 37 percent for Merkel’s conservative bloc.

Although the SPD aims to win sufficient votes to form a coalition with the Greens and possibly the radical Left party, most analysts think another “grand coalition” is the most likely outcome, with Merkel clinching a fourth term.

Under Gabriel, the SPD has pushed through several of its policy priorities, such as introducing a minimum wage in Europe’s biggest economy and reforming the pensions system. But with the migrant crisis and security fears now top of Germany’s political agenda, it is not winning over new voters.

Gabriel, the SPD’s longest-serving leader since Willy Brandt, was a protege of the last SPD chancellor Gerhard Schroeder and premier of the northern state of Lower Saxony between 1999 and 2003. He became SPD chairman in Nov. 2009.

In the last four years he has had to tread a delicate path between showing he can implement policies effectively in government and criticizing Merkel.

After the last ‘grand coalition’ with Merkel, the SPD slumped to a post-war low of 23 percent in the 2009 election.

Gabriel’s wife, Anke, a dentist, is due to give birth to their second child in March. SPD sources said he had an operation in December to shrink his stomach and help manage his diabetes.