Former Los Angeles Mayor Richard Riordan's push for a May ballot measure to cut pension benefits of city employees abruptly collapsed Monday, with a spokesman saying Riordan had suspended signature-gathering efforts.

Riordan's Save Los Angeles campaign had hoped to gather 300,000 signatures by Dec. 28 for a measure that would cut the pension benefits of existing employees and require new city workers to rely on a 401(k)-style retirement plan.

But according to a statement from spokesman John Schwada, "Riordan recently concluded that the Dec. 28 deadline cannot be met."

The statement said Riordan would explore other options "to accomplish the goal of pension reform."

"I ask the mayor, the city council and union heads to work with me over the next several months to save the city from bankruptcy and drastic cuts to public services," Riordan said.

The ballot measure proposal drew wide criticism from city employee unions, including the Police Protective League, which in recent weeks has sent out email blasts attacking Riordan.

Other city employee unions staged a vigil outside of Mayor Antonio Villaraigosa's house in protest of the proposed ballot measure.