So how can the IRS enforce the mandate? Scary letters and threats to withhold tax refunds.
The law allows the IRS to withhold tax refunds to collect the penalty, and most filers get refunds. This year, 77 percent of the 135 million individual income tax returns processed by the IRS qualified for a refund. The average refund: $2,707.
For those who don't qualify for a refund, a stern letter from the IRS can be effective, even if it doesn't come with the threat of civil or criminal penalties, said Elizabeth Maresca, a former IRS trial attorney who supervises the Tax & Consumer Litigation Clinic at the Fordham University law school.
"Most people pay because they're scared, and I don't think that's going to change," Maresca said.
The IRS has not yet issued procedures for taxpayers to prove they have insurance. But IRS Commissioner Douglas Shulman, in a 2010 speech, said he envisioned a process similar to the one used by taxpayers to report interest or investment income.
Under this scenario, an insurance company would send the taxpayer and the IRS forms each year verifying that the taxpayer has qualified insurance. Taxpayers would file the forms with the IRS along with their returns, and the IRS would check them to make sure they match the information supplied by the insurance companies.
The IRS says it is well on its way to gearing up for the new law but has offered little information about its long-term budget and staffing needs, generating complaints from Republican lawmakers and concern from government watchdogs.
The IRS is expected to spend $881 million on the law from 2010 through 2013, hiring more than 2,700 new workers and upgrading its computer systems. But the IRS has not made public information about its spending plans in the following years, when the bulk of the health care law takes effect.