The penalty for failing to maintain coverage is either a flat dollar amount or a specified percentage of household income, whichever is greater. For an individual with annual income of $40,000 and no coverage during the year, the penalty would be $741, according to a calculator on the I.R.S. website. For a couple with annual income of $90,000 and no insurance, the penalty would be $1,732.

Employers and insurers are supposed to file “information returns” identifying each person to whom they provide coverage. If they fail to do so, they too may be subject to penalties.

Aides to the Senate majority leader, Mitch McConnell, Republican of Kentucky, were working Monday on his bill to repeal major provisions of the Affordable Care Act, with the hope that they could meet objections from about one-fifth of his 52-member caucus. He has sent proposed revisions to the Congressional Budget Office for analysis.

To mollify moderate Republicans, he is considering restoring some money to Medicaid or keeping a tax on the investment income of the most affluent Americans. To satisfy conservatives, he is considering a proposal that would allow insurers to sidestep most federal insurance rules if they offer at least one health plan that complies with those standards.

Republicans in the Senate and the House generally agree on one thing: “The individual mandate has no place in a free society,” in the words of Representative Michael C. Burgess, Republican of Texas and a physician.

Many supporters of the Affordable Care Act, including Democrats in Congress, describe the individual mandate as an important part of the law. The law itself said the mandate would increase the number of healthy people buying insurance. Their premium payments help defray the cost of care for less healthy people and thus lower premiums in general, Congress said in 2010.

Using similar logic, the Congressional Budget Office said last week that repealing the individual mandate penalty, by itself, could lead to higher premiums. And many insurers cite uncertainty about the mandate as a reason for seeking rate increases for 2018.