BOSTON — Four cities including Boston could face downgrades in their bond ratings if state voters approve an expansion of charter schools, a major credit rating agency suggested this week.

Moody’s Investors Services contacted officials in Boston, Springfield, Lawrence and Fall River, warning that passage of Question 2 on the Nov. 8 ballot likely would be “credit negative” for those cities. A lowered credit rating can lead to higher borrowing costs for municipalities.

Critics contend school districts are losing more than $400 million this year to charters.

Eileen O’Connor, a spokeswoman for the pro-Question 2 Great Schools Massachusetts, a coalition of parents and educators, pointed to independent studies concluding that charter schools have “zero negative impact on district school finances.”

The Massachusetts Taxpayers Foundation, a nonpartisan fiscal research organization, recently reported that 3.9 percent of total education funding goes to charter schools, directly proportional to the 3.9 percent of public school students who attend charters.

“Question 2, with its financial and credit impacts, should concern every voter,” Democratic Lawrence Mayor Daniel Rivera said. “If Moody’s is concerned we should all be.”

Lawrence schools have been in state receivership since 2011.

Also Wednesday, the Massachusetts Charter Public Schools Association cited state data showing that children who attend charter schools in urban school districts do significantly better on standardized tests than those who attend traditional public schools in urban districts and in many cases do as well as those in suburban school districts.

Ten urban charters were ranked highest in the state on various assessment exams, matching scores posted by students in some of the state’s wealthiest suburban school districts.

The figures demonstrate “charter schools are closing the achievement gap between black and brown, low-income urban students and their white suburban peers,” said Shannah Varon, executive director of the Boston Collegiate Charter School.

The sides in the charter school debate have combined raised $37 million, campaign finance records show, making it the most expensive ballot question campaign ever in Massachusetts.

The Boston Globe first reported on the letter from Moody’s.

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This story has been corrected to show the charter school association cited test data Wednesday, not Thursday.