Published: May 17, 2010 10:47 AM EDT

WASHINGTON (AP) - Pitching President Barack Obama's health care

law to skeptical business owners, the IRS on Monday will announce

ground rules for small firms wishing to claim a new federal tax

credit for health insurance.

Created under the health overhaul law, the tax credit covers up

to 35 percent of the premiums that certain small businesses pay on

behalf of their workers. The IRS notice addresses unanswered

questions about the benefit, which is available starting this year.

The agency's action comes days after the nation's largest small

business lobbying group announced it was joining a lawsuit

challenging the health care law. The National Federation of

Independent Business argues that Congress overstepped its

constitutional authority by imposing a requirement that most

Americans obtain health insurance either through an employer, a

government program or buying it directly.

The IRS notice clarifies that employers can apply the credit

toward dental and vision benefits, not just medical coverage. A

fact sheet released by the Treasury Department also says that

employers can claim the federal benefit even if they receive state

tax credits for their insurance premiums.

The White House estimates up to 4 million small businesses may

qualify for the tax credit, but it's not clear how many will be

eligible. To begin with, they have to provide health insurance -

and many small employers don't. To qualify, companies must pay at

least 50 percent of their workers' premiums.

Eligibility is also limited by company size and wages. A firm

has to have fewer than 25 full-time workers averaging less than

$50,000 a year in pay.