Insurers say the change is needed to force companies such as Commerce Insurance, which pays less than its share of the high risk market, to pay their share.

But opponents say that such a balance could be achieved within the current system.

“You’d have very good drivers who happened to live in what the insurance industry calls bad territories [assigned to insurers] if you switched to an assigned risk plan,” said Ron Marlow, chief of staff for state Sen. Dianne Wilkerson, a legislator who opposes the change.

Insurance companies assign risk by neighborhood as well as by driving record, and black and low-income neighborhoods tend to be higher-risk zones.

Nearly 27 percent of drivers in the state’s high risk pool are in steps 9 and 10, the two best rankings in the state’s driving record rating system, according to industry data.

Opponents of the assigned risk plan worry that these good drivers would be assigned to companies and lose their freedom of choice if the plan is enacted.

To counteract such an occurrence, Attorney General Thomas Reilly recently introduced an amendment that would exempt drivers with clean records for the past three years from the assigned risk pool.

Commissioner Bowler has not yet made a decision on the insurers’ plan or on Reilly’s amendment.

“She is reviewing it,” said her spokesman, Chris Geotcheus.

Opponents of the plan say Reilly’s proposal does not go far enough.

“Although the attorney general’s plan is a good start, it can’t end there,” said Marlow. “Very good drivers in steps 12, 13 and 14 would still be included.”

Good drivers, including those in steps 9 and 10, are put in the Commonwealth Automobile Reinsurers pool due to factors such as where they live. Insurance companies also put inexperienced drivers and motorists in the pool if they do not want to write policies for them.

The state’s insurers contribute jointly to the costs of the CAR pool. The system allows drivers to choose any company they want, while insurers can transfer the costs of high-risk drivers to the pool.

The proposal put forward by the CAR governing body of insurers would eliminate the CAR pool all together.

The elimination of the pool would impact thousands of drivers in Roxbury and Dorchester. Forty-two percent of drivers in Roxbury are in the CAR pool, and 35 percent of motorists in Dorchester are in the pool — the two highest rates in the state.

Most drivers who are assigned to the pool do not know it. Opponents of the assigned risk plan say drivers assigned to new companies would have to deal with more paperwork and could lose discounts offered by their present insurers.

Commerce Insurance, the state’s largest insurer, plans to sue the state if Commissioner Bowler seeks to eliminate CAR, an entity created by the Legislature, without legislative approval.

A July 22 hearing, the only forum for public input on the plan, was dominated by representatives of insurance agencies, according to consumer advocate Stephen D’Amato, who attended “I think everyone assumed the whole thing was wired and the commissioner was going to approve it,” said D’Amato, executive director of the Center for Insurance Research. “But at the public hearing [July 22], the attorney general proposed a consumer protection plan that ameliorates all the problems of some people.”

Supporters of the assigned risk plan, which is similar to plans used in most other states, say that it would be the best way to ensure that all insurers pay their share of the high risk market.

Bowler’s spokesman, Chris Geotcheus, underscored the high cost to insurers for high risk drivers in Massachusetts.

“In 2003, the losses incurred by insurers for high risk drivers in the CAR pool was $380 million,” he said. “Switching to an [assigned risk] system would encourage insurers to reign in losses, some of which are undoubtedly fraud.”

The plan, which would be enacted in phases beginning in January and would only take full effect in 2007, is just one piece of Governor Mitt Romney’s push to reduce consumer protections and open up the state’s insurance market to more competition.

Romney has established a commission to look at introducing competitive rating to Massachusetts. Currently, the state sets the rates that all insurers must abide by.

The last time the state tried competitive rating was for six months in 1976, when soaring rates for inner city residents forced elected officials to backtrack and restore rate protections.