German politician Martin Schulz intends to take down Chancellor Angela Merkel

Former European parliament president and candidate for Chancellor of Germany's Social Democratic Party Martin Schulz delivers his speech during which he will be elected as its leader and lay out plans for the September general elections, on March 19, 2017 in Berlin. (Photo by AFP)

Martin Schulz on Sunday officially becomes Chancellor Angela Merkel's chief challenger in Germany's September general election and will lay out his plans for unseating the world's most powerful woman.

The Social Democrat, already credited with giving his ailing party a strong shot in the arm, will be anointed ther leader of the Social Democratic Party (SPD) and standard bearer at a one-day congress in Berlin.

German media predicted he could garner more than 90 percent of the vote from the country's oldest political party.

In a speech to the SPD rank and file, Schulz will attempt to harness his momentum against Merkel, whose conservatives just a few months ago had an apparently invincible lead in the polls.

Speaking on Saturday, Schulz said he hoped to win a vote of confidence with the backing of "a large majority" of party faithful.

His decision to leave the European Parliament, which he headed for five years, and be a candidate to lead Germany has given the Social Democrats a new lease of life since party leader 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," Schulz told Berlin public radio RBB this week.

Opinion surveys have recorded a 10-point jump for the SPD in recent weeks and some polls put it ahead of the conservative bloc of Merkel, who is trying to win a fourth term.

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