The number of the plaintiffs challenging the constitutionality of Germany's contribution has tripled since the end of June, claimed said the association Mehr Demokratie - More Democracy - on Tuesday.
Back in June more than 12,000 people, including ordinary citizens, eurosceptics from within Chancellor Angela Merkel's own party, and hardline Left party parliamentarians, petitioned the Constitutional Court to issue an injunction against the ESM.
They claimed that the creation of the bailout mechanism would undermine German lawmakers' constitutional right to decide on the budget and expose the country to potentially unlimited financial liability for ESM risks.
German President Joachim Gauck then declared that he would postpone ratifying the treaty until the Court had ruled on the constitutionality of the ESM.
In the meantime, support for the challenge has grown, says More Democracy, which was involved in petitioning for the injunction.
"After filing the first civil lawsuit, the flood of support continued to pour in unabated," CEO Roman Huber said. Since the initial filing, more than 25,000 people have added their names to the suit.
Merkel has continually stressed the importance of the court approving the European Stability Mechanism, which is intended to replace the temporary European Financial Stability Facility (EFSF). Were Karlsruhe to rule against the ESM, the EFSF would be left with just €150 billion.
German Finance Minister Wolfgang Schäuble said he was confident the court would not block the ESM.
"We don't have a plan B, and we don't need one," he said on Monday.
The judges are expected to issuing their ruling on 12 September.