An adviser at the European Court of Justice (ECJ) said on Wednesday that the scheme, which has never been used but is credited with stabilising the euro, fell within the ECB's remit.
That appeared to contradict the view of Germany's Constitutional Court, which last year said there was reason to think it went beyond the bank's mandate.
AfD leader Bernd Lucke said that if the ECJ followed the adviser's opinion - which is usually what happens - it would be in "serious conflict" with Germany's Constitutional Court in Karlsruhe, southwestern Germany.
"The judges in Karlsruhe would then consider the ECJ's ruling tantamount to contempt of the (EU) Treaties so they'd have to conclude that the euro is no longer a community of stability," he was quoted as saying by Handelsblatt.
"According to the jurisprudence of the Constitutional Court, this would open the possibility of Germany leaving the euro zone," said Lucke, who is an economics professor.
Also an outspoken critic of bailouts for struggling euro zone member states, Hans-Werner Sinn, head of the influential Ifo institute, said the courts' positions were irreconcilable and warned the euro zone would descend into a "major constitutional crisis" if the ECJ adopted the adviser's opinion.
ECB President Mario Draghi announced the so-called Outright Monetary Transactions (OMT) programme at the height of the euro zone crisis in September 2012. Germany's Constitutional Court referred a complaint against the scheme to the ECJ last year.
However, the German government welcomed the ECJ opinion, which eliminates a major hurdle to the ECB's plans to boost the flagging euro zone economy by buying up sovereign debt. But some conservatives expressed dismay.
Hans Michelbach, from Chancellor Angela Merkel's conservative bloc, said opening the door to bond-buying removed the incentive for struggling euro zone states to implement reforms and broke the taboo of financing governments.
"The ECB would turn into a bad bank and a serious danger for the consolidation of budgets in euro zone states. This is the beginning of a transfer union by the back door," he said, referring to concerns in Europe's largest economy that its taxpayers are funding weaker euro zone member states.
Peter Gauweiler, a conservative lawmaker who was a plaintiff in the original case in Germany, argued in Handelsblatt the ECB would have to leave the "troika" of international lenders that check reforms in bailed-out countries if it bought state debt.
(Reporting by Michelle Martin; Editing by Stephen Brown and Louise Ireland)
