HARTFORD, Conn. — Connecticut’s largest association of cities and towns has launched an online advertising campaign, urging state lawmakers to pass legislation the group says will protect municipalities from expensive bond rating downgrades.

A bill awaiting Senate action prohibits an arbitrator from considering a municipality’s budget reserve account when determining its ability to pay labor contracts, so long as the reserves represent 15 percent or less of the municipality’s annual budget.

The Connecticut Conference of Municipalities announced its online ad campaign on Tuesday, arguing the bill will help preserve a municipality’s bond rating by allowing it to maintain a level of reserves recommended by credit-rating agencies. CCM says the bill also protects those reserves for unanticipated expenses, such as natural disasters and reductions in state aid.

Unions say the bill could hurt workers.