PORTLAND — The Maine Public Utilities Commission will hold a public hearing tonight on a proposed $8.63 million increase in Unitil’s distribution base rate that would affect thousands of natural gas customers in Greater Portland.

It would be the first distribution rate increase in 28 years for Unitil, which purchased Northern Utilities in 2008. There have been numerous increases in the second component of the company’s billing, for the gas itself, over that period, but those supply rates have been down significantly in the past several years.

State Rep. Ben Chipman is so concerned about the new proposal that he is offering rides to the hearing for anyone who lives in his downtown Portland district. He will ask the commission to postpone any distribution rate increase until after the upcoming winter heating season has ended.

“The timing couldn’t be worse. As we begin the cold winter season, this rate increase would have a chilling impact on low-income families that are already struggling to make ends meet and can barely afford to heat their homes right now,” said Chipman, who represents Portland’s Parkside, Bayside and East Bayside neighborhoods.

Tonight’s hearing is scheduled to begin at 5:30 p.m. in the Abromson Center at the University of Southern Maine in Portland.

According to the PUC’s website, Unitil’s proposal would have the effect of increasing the rates of residential and small commercial customers proportionately more than larger commercial and industrial customers.

George Gantz, Unitil’s senior vice president, said that after the 2008 Northern Utilities purchase, Unitil agreed to postpone any rate increases for at least two years.

Besides covering current operating expenses and investments in facilities and equipment, the company said the new rates would help fund the recently begun effort to upgrade more than 100 miles of pipe in Portland and Westbrook. The 14-year project is estimated to cost $60 million.

Unless the PUC acts, a three-phase increase will go forward. All customers would see slightly higher bills starting Nov. 1, a larger increase as of Jan. 1, and the final increase on May 1.

For customers who heat with gas, the change amounts to a 15.8 percent increase in their annual heating costs, or $20.48 per month for a typical customer. For households using gas for non-heating purposes, such as cooking, the average increase would be $6.47 per month.

Unitil serves about 26,000 customers in 22 communities that include Portland, Westbrook, Cape Elizabeth, Scarborough, South Portland, Biddeford, Saco, Wells, parts of Sanford and Old Orchard Beach.

No final decisions will be made by the PUC tonight.

Staff Writer Dennis Hoey can be contacted at 791-6365 or at: dhoey@pressherald.com