The German vice chancellor shook up the election on Tuesday by withdrawing as the Social Democratic challenger to Angela Merkel. He will likely be replaced by Martin Schulz, the former president of the European Parliament, who may be nominated Sunday.

Sigmar Gabriel, the pugnacious but unpopular head of Germany's Social Democratic Party, said Tuesday he will not run as his party's chancellor candidate against Angela Merkel in national elections this fall.

Mr. Gabriel's announcement, which will shake up the German race, was made to weekly Die Zeit, a sister publication of Handelsblatt Global.

The 57-year-old Gabriel said he will relinquish his post as vice chancellor and head of the SPD, Germany's second-largest party and the junior partner in Ms. Merkel's coalition.

He plans to take on the job of German foreign minister, which will become vacant after its current holder, Frank-Walter Steinmeier, becomes president.

Mr. Gabriel's surprise departure clears the way for Martin Schulz, the former president of the European Parliament who recently returned to Germany to resume a domestic political career, to take on Ms. Merkel.

A majority of SPD voters in a recent survey preferred Martin Schulz, the former president of the European Parliament, to Mr. Gabriel as their chancellor candidate.

Mr. Gabriel's decision to step down as his party's chancellor candidate in this September's election came after recent polls showed his party trailing Ms. Merkel's Christian Democratic Party.

His centrist positions -- endorsing an aborted trans-Atlantic trade agreement with the United States for example -- were unpopular in his own party, which is increasingly dominated by a left wing that has been shut out of power for 11 years.

"Gabriel's decision appears to be the conclusion of the fact that he can no longer convincingly do the splits between being a loyal government employee and a serious challenger of Merkel at the same time," Nils Diederich, a professor at Berlin’s Free University who focuses on Germany's party political system, told Handelsblatt Global.

"Mr. Schulz is the sole alternative," Mr. Diederich added. "He has a clean slate when it comes to domestic policy and is a clear Merkel challenger, as he represents the only convincing alternative to the CDU/CSU."

A recent survey of SPD supporters, which Mr. Gabriel apparently commissioned himself, appears to be what tipped the scale for his decision to step down. A large majority of respondents seemed to think Mr. Schulz, who built a reputation as a verbose but effective partisan leader in Brussels, had a better chance of beating Ms. Merkel this fall.

"There are two basic prerequisites for a successful election campaign: The party has to believe in the candidate and gather behind him, and the candidate himself has to want it with every fiber of his heart." Mr. Gabriel told Stern magazine in an interview, adding he did not meet either condition.

Gabriel, who will become a father again in March, also cited "private reasons" for his decision.

"Today I am really a happy person. I don't know if this would be the case if I saw my family even less than I do already," he said in a statement.

Video: Martin Schulz resigning as European Parliament president on November 24, 2016, in Brussels.

Mr. Gabriel told Die Zeit that he would be communicating this decision to the party's leadership this evening. He said he intends to support Mr. Schulz's candidacy and will communicate this to SPD members when the party meets on Sunday in Berlin to approve a nominee.

The decision is certain shake up Germany's already turbulent election year, in which Ms. Merkel, wounded politically by the Christmas terror attacks in the wake of her open-door refugee policy, is seeking a fourth term in office.

Before Mr. Gabriel's departure, most German political experts anticipated a continuation of the CDU-SPD "grand'' coalition in September. According to a poll by INSA on a German political website wahlrecht.de, the CDU leads all parties in polling ahead of the September election, with 32.5 percent of likely voters. The SPD trails with 21 percent, and the upstart Alternative for Germany Party, which opposes immigration, is third with 14 percent.

Mr. Schulz's elevation to become the SPD chancellor candidate is likely to reinvigorate the party's base, and could boost its standing among voters going forward. It remains unclear, however, whether the change in top candidates will be enough to strengthen the SPD's own desire to unseat Ms. Merkel and lead a coalition that could include the Green and even the Left Party.

In the INSA poll, the Greens had 8.5 percent and the Left Party 11 percent. Together with the SPD the three parties still come up short of the 50 percent minimum needed to build a governing coalition.

Unclear is also whether national support for the Alternative for Germany is understated, with voters uneasy about publicly identifying with the party -- one of whose members is being investigated for making public remarks widely considered to be antisemitic.

As in Britain before its Brexit vote last June, and in the United States before its election of Donald Trump in November, pollsters failed to detect a groundswell of dissatisfaction with the status quo and political establishment.

In a perhaps eerie parallel, experts in Germany are echoing sentiments that Ms. Merkel is mathematically unbeatable and the only option is a continuation of the status quo.

Kevin O'Brien is editor in chief of Handelsblatt Global. Tina Bellon and Daniel Tost are editors at Handelsblatt Global in Berlin. To reach the authors: [email protected].