BOSTON – America’s most comprehensive health reform effort on yesterday became law as Governor Romney signed a bill that will assure near-universal health insurance for Massachusetts residents.

The measure followed a year of negotiations between the Republican governor and the overwhelmingly Democratic state legislature, and was swiftly heralded as a national model. Insurance providers and health experts also took part in deliberations that will require all state residents to purchase health insurance by July 2007.

“An achievement like this comes around once in a generation, and it proves that government can work when people of both parties reach across the aisle for the common good,” Mr. Romney said yesterday as he signed the bill in a ceremony at historic Faneuil Hall.

“Today,” the governor said, “Massachusetts is leading the way with health insurance for everyone, without a government takeover and without raising taxes.”

But Mr. Romney exercised his line item veto power to overturn eight portions of the bill, including a $295-per worker assessment on businesses that was seen as a critical piece of the bill.

Some critics described the fee as a tax on business, and in call-in radio shows over the weekend many small business owners told the governor that the assessment was an onerous burden. In vetoing the provision, Mr. Romney said the fee is “not necessary to implement or finance health care reform.”

The governor also vetoed a section of the bill that would have extended dental benefits to adult Medicaid recipients, at an annual cost of $75 million.

Legislative leaders have vowed to override Mr. Romney’s vetoes.

The law targets more than half a million state residents who currently have no health insurance. Using a sliding scale, low-income residents will be able to obtain health insurance at greatly reduced costs – and in some cases, at no cost.

State residents who can afford private insurance will face tax penalties if they do not obtain coverage.

The law spreads costs and administrative responsibilities among citizens, employers and government. But one incentive for drafting the legislation was a brief window in federal funding that would provide $385 million in annual subsidies if the state could reduce its uninsured population.

Private insurance companies will be eligible for government subsidies to increase coverage for children and the working poor.

The measure also creates a health insurance “connector” to link businesses and individuals with insurance providers.

The bill is expected to cost $316 million the first year, rising to more than $1 billion by the third year. Much of the money will come from federal reimbursements and existing state spending.