WASHINGTON - Millions of immigrants benefiting from President Barack Obama's executive actions could get a windfall from the IRS, a reversal of fortune after years of paying taxes to help fund government programs they were banned from receiving.

Armed with new Social Security numbers, many of these immigrants who were living in the U.S. illegally will now be able to claim up to four years' worth of tax credits designed to benefit the working poor. For big families, that's a maximum of nearly $24,000, as long as they can document their earnings during those years.

Some Republicans are labeling the payments "amnesty bonuses," one more reason they oppose Obama's program shielding millions of immigrants from deportation.

"I represent hard working, law-biding Texans," said Rep. Sam Johnson, a senior Republican on the House Ways and Means Committee. "I think these amnesty rewards, and that's what they are, need to be stopped."

Advocates argue that many of these immigrants pay taxes, so they should be able to claim the same tax credits as anybody else. Over the past decade, immigrants in the U.S. illegally have paid an estimated $100 billion in Social Security payroll taxes, even though few will ever be able to collect benefits, said Stephen Goss, Social Security's chief actuary.

Obama has issued executive orders shielding about 4 million immigrants from deportation. Some were brought to the U.S. as children; others are parents of children who are either U.S. citizens or legal residents.

Republicans in Congress oppose Obama's actions and are trying to use a funding bill for the Department of Homeland Security to overturn them. Democrats are resisting, resulting in a stalemate that is threatening to shut down the department.

Funding for the department, which oversees immigration enforcement, runs out Feb. 27.

The dispute over tax credits illustrates the complicated relationship that many immigrants have with the U.S. tax system. Social Security estimates that immigrants living in the country illegally work at about the same rate as the rest of the population, even though federal law bars them from employment.

In general, they are less likely to pay federal taxes. Those who do have been boosting Social Security's finances for years.

How does Social Security know when it receives taxes from immigrants who are in the U.S. illegally? There is no foolproof method, Goss said. One way is by tracking reported wages in which the Social Security number does not match the name the agency has on file.

Some of these are clerical errors or unreported name changes, But Goss estimates that a majority of these wages come from immigrants who have made up Social Security numbers or used someone else's.