Massachusetts began signing up its poorest residents for low-cost health insurance, the first step in the state's goal to be the first in America to require all citizens to have health insurance.

"This is a historic day for us," said Gov. Mitt Romney on Monday. "It's real today."

Romney signed the state's new health care law in April.

This summer, the federal government gave the state its approval for a first-in-the-nation program that will require everyone 18 and older to carry health insurance. The state's universal health insurance program will use a combination of subsidies and penalties to make coverage more affordable and to encourage people to buy it.

Romney officials used the news conference to press lawmakers to close what they called a "loophole" in the law that fails to require all children to have health insurance.

"If there are affordable products out there, all the evidence suggests that parents will cover their children," Health and Human Services Secretary Tim Murphy said.

Murphy said as many as 40,000 children might fall through the holes, but lawmakers said they have already taken steps to expand coverage for children.

Next July, all Massachusetts residents earning more than three times the federal poverty level will be required to have health insurance — on their own or through work — or face tax penalties.