Mitt Romney's top rivals are reminding voters that Massachusetts residents have until Thursday to sign up for health insurance or face possible penalties -- a requirement that Romney signed into law when he was governor.

The Republican presidential candidate isn't doing as much to mark the milestone.

Massachusetts residents had to sign up by Nov. 15 or they likely would face tax penalties starting Jan. 1. It is the stick that follows the carrot of previous deadlines requiring the state to expand subsidized coverage, or for private insurers to offer less expensive policies to the uninsured.

It's also something opponents for the Republican nomination are trying to turn it into a political liability for Romney.

Such mandates are anathema to fiscal conservatives and other bedrock GOP voters who oppose government intrusion, explaining the silence by Romney and the criticism from former New York Mayor Rudy Giuliani and other candidates also vying for conservative votes.

"Before I forget, for all of you in Massachusetts who aren't signed up for health insurance, you have until Thursday before you get stuck with a fine," Maria Comella, Giuliani's spokeswoman, wrote in signing off a daily schedule update for political reporters.

An aide to former Tennessee Sen. Fred Thompson e-mailed audio of a public service announcement reminding Massachusetts residents of the deadline, as well as the tax consequences, and sought campaign contributions for Thompson.

A third candidate, Sen. John McCain of Arizona, hasn't focused on the deadline as much as the mandate itself. The senator recently told a health care forum that while he favored universal college education, he would not require it.

"I feel the same way about health care," McCain said.

Romney chuckles at the criticism, and then offers a serious comeback.

"I am the only Republican in this race that's actually succeeded in getting all of the citizens of my state on track to have health insurance," he told The Associated Press on Tuesday during an interview in Iowa. "That's a good thing, and I'm happy to defend my plan."

The law aims to reduce the ranks of uninsured in Massachusetts, a group once estimated at up to 500,000. As of July 1, all state residents had to be insured either through government programs or private insurance provided by their employers, subsidized by the state or driven down in cost through state collaboration with private insurers.

They did not face penalties, though, unless they continued to be uninsured after Dec. 31. And Thursday is the last day many insurers will take on new customers and cover them as of Dec. 31. Some say they will continue to sign up customers until New Year's Eve.

Anyone lacking coverage on Dec. 31 will lose the personal exemption on their state income tax filing next spring, equal to $219.

If they remain uninsured into 2008, they will be taxed up to 50 percent of the cost of the least-expensive private insurance plan -- an estimated hit of at least $150 a month.

The mandate is a critical element of the new law, since it aims to stop uninsured "free-riders" from walking into an emergency room and sticking the insured with the bill through higher premiums. Romney likened it to the state's auto-insurance requirement, and pitched it as a matter of personal responsibility when he signed the health insurance bill into law in April 2006.

The law appears to be working. As of Nov. 1, the date for the most recent statistics, more than 200,000 formerly uninsured people had gotten insurance, roughly half of the state's target.

Nonetheless, Romney has distanced himself from the mandate.

The national health insurance provisions he unveiled earlier this year specifically avoid a mandate. Romney has said he would leave it up to the states to devise their own plans. He later criticized Democratic front-runner Hillary Rodham Clinton for including a mandate in her health care proposal.

Romney said he thinks the Massachusetts plan enhances his appeal for support from conservatives.

"I think mine is the ultimate conservative approach," he said. "The good news is we've proved that we can get everybody insured without the government handing out government insurance, and without spreading Medicaid to everybody, and without a government takeover of health care. ... I'm the only guy whose got a free-market way to get everybody insured."