The amendment would reduce the number of city aldermen in St. Louis to 14, and the number of wards from 28 to seven.
The move comes one day after a related bill easily passed the Missouri House.
State Senator Maria Chappelle-Nadal (D, University City) supported the amendment in her overall opposition to the bill.
"This has everything to do with pensions and (with) taking away the employment terms and benefits from police officers," Chappelle-Nadal said. "If it starts with police officers, it will spread to teachers and to firefighters."
The bill's sponsor, State Senator Joseph Keaveny (D, St. Louis), says the amendment was specifically added to make the bill harder to pass.
"There's no doubt about that," Keaveny said. "The amendment's completely unrelated to the issue...it's an attempt to fracture the strong coalition (that favors the bill)...obviously with that amendment on it, it's not quite so popular."
Keaveny expressed hope that the amendment will eventually be removed. Meanwhile, Senate opponents to local control may also try to block the bill via filibuster.