This is high season for budget blueprints on Capitol Hill.

The week began with the unveiling of the Obama administration’s budget request for the upcoming fiscal year. Meanwhile, in Congress, lawmakers are in the midst of debating a spending measure to provide current year funding for the federal government. As the action unfolds, the burning question is whether Republicans will try to “defund” health care reform and, if so, what strategy might they employ to do it. In fact, the defunding has already begun.

A key provision of the health law that will provide 19 million consumers with tax credits to help afford their health insurance has been raided once. Now, Republicans are planning to raid it again.

Here’s the background: In late December Congress was looking for money to help pay for a plan to stop dramatic scheduled cuts in doctors’ Medicare reimbursements. As a temporary fix, the lame duck Congress changed a part of the new health law so that some consumers will have to pay back a hefty chunk of their tax credits. That change is expected to save the government $16 billion over 10 years.

Now Congress is again looking for money – this time to offset the funds lost with the repeal an unpopular health law provision that involves 1099 tax reporting requirements. Republicans are proposing another increase in the liability of tax credit recipients to raise the necessary revenue.

To understand why the proposal is worrisome, it is necessary to understand the function of the new tax credits. They are designed to help finance insurance premiums beginning in 2014, and are, perhaps, the most important provision in the new law for making health care affordable to middle-class Americans. They will be available on a sliding scale to families with incomes up to 400 percent of the federal poverty level. This is $88,200 for a family of four, but most of the money will go to families with far less income.

Here’s how they work. The amount of the tax credit for which people are eligible is based on annual income for the year the credits are received. But, because people will likely need help paying their insurance premiums during that tax year, the law provides for advance payments of the credits. These payments are made directly to the insurer on a monthly basis. The insurance exchanges, through which individuals will purchase health coverage, will determine eligibility for tax credits based on a taxpayer’s prior year tax return. (Taxpayers can provide more recent information if their income or family circumstances have changed.) At the time taxpayers file their annual tax return, the advance payments will be reconciled against the tax credit for which the taxpayers are eligible using their annual income reported on their return. If the advance payments are greater than the final tax credit, the taxpayer will get a bill from the Internal Revenue Service.

Excess advance payments can happen easily and will happen often. The income of hourly-wage lower and middle-income Americans often fluctuates from week to week and is difficult to predict. Dependents may leave or return home. Family members may become eligible for Medicaid or CHIP. Taxpayers may be eligible for a premium tax credit in the early months of the year while unemployed but then get a job with coverage and no longer need premium assistance. Or they may lose a job part way through the year and face dramatically reduced income, even though their full year reported income remains high.

All these changes will affect the subsidy calculation. It will be difficult for the exchanges to keep up with changes in family circumstances and for families to know what changes they should report and to whom. It is inevitable that there will be some inconsistency between advance payments based on estimated income for the year and the final credit determined at tax time.

Originally, when the health overhaul was signed into law, the amount the government could recover was capped at $400 for families with incomes below 400 percent of poverty. The amendment adopted in December increases the amount families will owe on a sliding-scale basis. Under the December amendment families with incomes at 200 percent of poverty will have to pay back as much as $1,000; families with incomes at 400 percent will have to pay back up to up to $2,500. It also, however, puts some limits on overpayments for families up to 500 percent of poverty.

The “1099 fix” legislation, which is likely to be considered this week by the House Ways and Means Committee, would require families at 200 percent of poverty to pay $1,500, and families earning a dollar more than 400 percent of poverty to pay back their entire tax credit.

It is important to understand what is at stake here.

Fear of potential end-of-year liability could be a substantial deterrent to participation in the advance premium tax credit program. It was estimated that the December amendment increased the likely number of uninsured after 2014 by about 200,000 people, who would rather be uninsured than face substantial repayments. Millions more consumers will face unanticipated financial burdens. This is likely to create a powerful backlash, as Americans who thought they were receiving a tax credit to help them purchase insurance find out it was in fact only a loan, and that they owe the IRS a substantial debt.

These changes are not needed to deter any potential gaming of the system. The health law provides separately for substantial fines for providing false information, even negligently. The December-passed amendment simply increased the penalty for failing to accurately make a very difficult projection of future income and family circumstances, and the new proposal would increase the penalty even more.

The December amendment was a compromise, improving some aspects of the legislation while diminishing tax credit protection. But it also set a bad precedent by cutting premium subsidies before they even become available. It can only be hoped that Congress does not adopt the proposed amendment, again increasing the risk of Americans who accept premium tax credits, as it attempts to find ways to cut the budget during this session.