German's constitutional court is hearing arguments Tuesday in a legal challenge to a key European Central Bank program that helped quell market panic during the eurozone debt crisis.

The plaintiffs, including a conservative former lawmaker and a citizens' group, are challenging the ECB's offer to purchase government bonds of countries facing excessive borrowing costs. They say it breaches limits on using central bank powers to help finance government spending.

The European Court of Justice upheld the bond-purchase offer last year and sent the case back to the German court.

Even if the German judges reject that ruling, they lack jurisdiction at the European level. They could hinder the offer by barring Germany's central bank from taking part.

The offer has never been used. A decision is expected in coming months.