Olaf Scholz, the centre-left Social Democrat (SPD) candidate to succeed Angela Merkel, is often described as boring, but could be on the verge of a sensational upset after Sunday's election.

With polls showing the SPD narrowly ahead of Angela Merkel's CDU-CSU conservative alliance, Scholz may have achieved something many would have thought impossible just a year ago.

His SPD scored just 20.5 percent in Germany's last election in 2017 and has had a difficult few years in coalition with the CDU-CSU, but looks on course to win at least 25 percent of the vote this time.

"It's going to be a long election night, that's for sure," Scholz said after the first estimates were released.

"But this is certain: that many citizens have put their crosses next to the SPD because they want there to be a change in government and also because they want the next chancellor to be called Olaf Scholz."

Nicknamed "Scholzomat" for his robotic speeches, Scholz is one of Germany's most influential politicians, with a reputation for being meticulous, confident and fiercely ambitious.

- Channelling Merkel -

As finance minister and vice-chancellor under Merkel, he enjoys a close relationship with the chancellor and has even sought to position himself as the true Merkel continuity candidate, despite hailing from a different party.

He was pictured recently on the cover of the Sueddeutsche Zeitung magazine adopting Merkel's famous "rhombus" hand gesture -- a stunt that provoked consternation from rivals in Merkel's CDU camp.

During his time as finance minister, Scholz has cemented his reputation for being on the fiscally conservative side of his workers' party.

Despite agreeing to suspend Germany's cherished "debt brake" to stave off the crippling effects of the coronavirus pandemic, he has insisted on a return to the policy by 2023.

"All this is expensive, but doing nothing would have been even more expensive," he said at the time.

Scholz's cautious approach has at times seen him marginalised within the SPD, overlooked in a leadership vote in 2019 in favour of two relatively unknown left-wingers.