The latest indicator of gay people's growing economic clout came last week in a report that found more than half of the United States' largest corporations now extend health insurance to employees' same-sex domestic partners.

The Washington-based Human Rights Campaign Foundation found that the number of Fortune 500 companies offering this benefit has doubled in six years, even as voters in 45 states have registered their opposition to gay marriage.

Moreover, the report found that large companies have more readily granted benefits to their workers' same-sex partners than have state and local governments. More than 250 companies are offering that benefit this year.

"They are light-years ahead because corporate leaders are making these decisions purely on a business model," said Joe Solmonese, the foundation's president. "Looking at the buying power of the lesbian and gay communities and the need to attract a diverse and talented work force, they've concluded that a fair and inclusive work force serves them best."

The tally of cities, counties and other government organizations providing health benefits to same-sex partners rose to 201 this year, up from 113 in 2000. Eight states grant some legal rights, including access to insurance benefits, to same-sex couples.

These findings echo similar results from Mercer Human Resource Consulting's annual benefits survey, said Kathleen Murray, a principal in the company's San Francisco office. It's all about talent, she said, "and getting the right people in."

Job candidates increasingly ask about domestic-partner benefits, Ms. Murray said, and companies that don't provide them often find that the interview "falls flat."

Human Rights Campaign's "State of the Workplace" report is the group's seventh annual survey. It also found that 86 percent of Fortune 500 companies now bar discrimination based on sexual orientation.

But the findings on domestic-partner benefits were the most striking, experts said. Mr. Solmonese said companies reported the benefit added "less than 1 percent to their total benefits costs" -- a relatively small price to pay for being able to attract diverse employees and customers.

Citing 2000 census figures showing that same-sex couples live in nearly every U.S. county, Mr. Solmonese said many of these consumers base their buying decisions on a company's workplace policies. The report estimates that lesbian, gay, bisexual and transgender customers are a $641 billion market.

Some conservative groups oppose the extension of health benefits.

Steve Crampton, chief counsel for the American Family Association's Center for Law and Policy, called the benefits "a bad trend for America, encouraging dangerous and demonstrably destructive behavior."

Nationwide Mutual Insurance Co. doesn't see it that way. Since 1999, the Columbus, Ohio-based company has offered its 35,000 employees benefits for same-sex partners, as well as for aging parents, adult children and roommates.

"As the traditional household has changed, we decided to let our associates define family," spokesman Eric Hardgrove said.

When Ohio voters amended the state's constitution in 2004 to define marriage as a union between one man and one woman, Nationwide Mutual didn't change its policy.

Mr. Hardgrove said the benefits plan "goes along with our commitment to building a workplace that fosters respect, is inclusive and nondiscriminatory."