Martin Schulz on Sunday, March 19 officially became Chancellor Angela Merkel's chief rival at Germany's September general election, leading the leftwing challenge to unseat arguably the world's most powerful woman, AFP reports.

The bearded Social Democrat, already credited with giving his ailing party a strong shot in the arm, was unanimously elected SPD leader and standard bearer at a one-day congress in Berlin.

In a speech to the SPD rank and file, the 61-year-old attempted to harness this momentum against Merkel, whose conservatives just a few months ago had an apparently invincible lead in the polls.

"Starting now the fight begins to become the top party in the country and take over the chancellory," Schulz told the party congress.

Schulz's decision to leave the European Parliament, which he headed for five years, and become a candidate for German leader has given the Social Democrats a new lease of life since ex-party chief Sigmar Gabriel asked him to take the reins in January.

"It's been encouraging to see in the last few weeks that people are hopeful again that the Social Democrats have a shot" at election victory, Schulz told Berlin public radio RBB this week.

"My intention to pursue policies that make the lives of hard-working people a little better is apparently finding a lot of support."

Opinion polls have recorded a 10-point jump for the SPD in recent weeks with some placing it ahead of the conservative bloc behind Merkel, who is hoping to win a fourth term in office.

The SPD congress has fired the starting gun for the national election campaign and also the race for three German state polls, the first of which will be held in Saarland on the French border, on March 26.