The Brazilian city is due to host five matches at the tournament, which starts on June 12, and the Estadio Beira-Rio is nearly ready.
But temporary facilities for media, sponsors and other FIFA requirements are still needed for it to be completed.
Fortunati said a bill creating tax exemptions for companies to invest in the temporary structures needed to be passed on Tuesday.
"If the (project) isn't voted then there won't be a World Cup (match) in Porto Alegre," Fortunati told Radio Gaucha.
"We don't have a way to get that funding. There's no Plan B, no Plan C, no Plan D. This is the only option."
Public money can only be used on structures which will be utilised after the World Cup finishes on July 13.
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