Germany should be legally bound to invest in infrastructure, the centre-left challenger to Chancellor Angela Merkel at September elections said Sunday.

Berlin "must use its money to improve public infrastructure according to binding rules," Social Democratic Party (SPD) candidate Martin Schulz said at a campaign event in the German capital.

Schulz did not call for the lifting of the "debt brake" built into the German constitution by Merkel's conservative government in 2009, which sets legal limits on deficits in federal and regional budgets.

Berlin has booked budget surpluses in recent years and reduced its debt levels under finance minister Wolfgang Schaeuble.

German public spending is a hot topic at home and abroad. Trading partners have called on the government to invest more as a way of reducing its massive trade surplus -- the amount its exports outweigh its imports.

Countries like France or the United States argue that while Germany is happy to rake in cash from selling its goods abroad, it fails to help other economies by spending at home to contribute to economic growth.

French President Emmanuel Macron said in an interview published Thursday that Germany had to move on investment.

And The Economist magazine labelled trade surpluses "the German problem" on its latest cover. It accused the country of saving too much and spending too little alongside an image of the imposing German eagle -- its wings incorporating a graph of trade statistics.

"The state can't create any illegal deficits -- and that's right," Schulz said Sunday.

But the SPD argues that increasing investment is a matter of "intergenerational fairness", slotting it into a 10-point programme alongside improving social justice and a stronger European Union.

The plan would oblige the state to spend on high-speed internet connections, transport links, renewable energy and education, especially in the country's economically weakest regions.

It stands alongside other manifesto commitments including improved wages, fewer insecure jobs and free education up to masters degree level, as well as tax cuts for families.

Schulz, a former president of the European Parliament has a formidable opponent in Merkel.

She has regained a commanding lead in opinion polls after Schulz enjoyed a brief surge that took the SPD within touching distance of the Chancellor earlier this year.