BRUSSELS — Martin Schulz, the president of the European Parliament since 2012, announced on Thursday his return to German politics, fueling speculation that he might challenge Chancellor Angela Merkel, who plans to seek a fourth term in elections next year.
The announcement by Mr. Schulz, 60, had been expected. Speculation about his return to German politics gained new momentum after Frank-Walter Steinmeier, Germany’s foreign minister, was tapped to become the country’s next president.
Mr. Schulz said he would seek a seat in the Parliament from North Rhine-Westphalia, the most populous state in Germany. From there, he might lead the Social Democratic Party’s challenge to Ms. Merkel’s party, the center-right Christian Democratic Union, in elections scheduled for September. The two parties have governed Germany in a “grand coalition” since 2013.
Another possible scenario is that Mr. Schulz would replace Mr. Steinmeier as foreign minister, a possibility if the Social Democrats were to select their chairman, Sigmar Gabriel, currently the vice chancellor, as the party’s candidate for chancellor, the head of government.
A former bookshop owner and mayor of Würselen, a town in North Rhine-Westphalia, Mr. Schulz was elected to the European Parliament in 1994. He rose to prominence in 2003 when Italy’s prime minister at the time, Silvio Berlusconi, called him a “capo,” or a Nazi concentration camp commander, during a debate at the European Parliament.
Mr. Berlusconi made the insult after Mr. Schulz questioned him about conflicts of interest in his business affairs. Mr. Schulz was warmly applauded by other lawmakers after describing Mr. Berlusconi, a tycoon, as unfit to represent Europe.
On Thursday, Mr. Schulz was circumspect about his plans and declined to take any questions from reporters. His decision also left open the question of who will succeed him as the president of the European Parliament.
The two biggest political groups in the Parliament, Mr. Schulz’s Social Democrats and the center-right European People’s Party, have long had an informal arrangement in which they take turns at the presidency.
Unusually, a socialist, Mr. Schulz, has held the job for the last two terms, and he was originally thought to be interested in a third.
Mr. Schulz’s departure makes it more likely that a member of the center-right would get the job. But that would put a member of the center-right in charge of the top three European Union jobs. Donald Tusk, the president of the European Council, the body that represents national leaders, and Jean-Claude Juncker, the president of the European Commission, the bloc’s executive body, are both from the center-right.
To preserve political balance at the top of the European Union, another socialist, or even a member of the Alliance of Liberals and Democrats for Europe, could succeed Mr. Schulz when the full Parliament votes for the next president in mid-January.
“From now on I will be fighting for this project from the national level, but my values do not change,” said Mr. Schulz, who has used his presidency of the only directly elected body in the European Union to widen its influence over many areas of European policy-making.
“I will continue to do my utmost to improve people’s everyday life, to narrow inequalities within and between societies. Only by doing this will it be possible to regain the lost trust,” Mr. Schulz said, in an apparent reference to the populist parties that have roiled politics across the Western world, including parts of Germany.
“Germany in particular as the biggest member state of the European Union bears a special responsibility which until now I have tried to fulfill as a German member of the European Parliament and in the future I will strive to fulfill from Berlin,” he said.