The debate over the Senate's comprehensive immigration reform bill has forced lawmakers to grapple with another contentious issue, namely: What exactly are the fiscal consequences of granting citizenship to the 12 to 15 million illegal immigrants living within our borders and the millions more who yearn to settle here?

Will they strive, like modern-day Horatio Algers, to surmount the abject poverty they experienced in their homelands? Will they bypass our dependency-inducing welfare state and raise their children within self-reliant, two-parent families? Will their offspring ascend the socioeconomic ladder, live the American Dream, and ultimately become tax payers rather than tax consumers?

Or will they tap the vast array of government programs that provide even the hardest working among them with subsidized health care, housing, nutrition, education, and retirement benefits, eventually becoming fiscal drains on the American taxpayer?

Proponents of the Senate's so-called "grand bargain" insist that the latest wave of immigrants will be net contributors. "If they work hard and play by the rules and work in very tough and menial jobs," the bill's author, Sen. Ted Kennedy (D-Mass.) told his colleagues, "they may have an opportunity to be a part of the American dream." Sen. Robert Menendez (D-N.J.) believes the immigrants' "entrepreneurial spirit" and "incredibly hard work ethic" gives the economy "a much needed boost." For many Americans, the real "grand bargain" was summed up by a New Jersey man interviewed in The New York Times: "If they have been working here and are law abiding and can contribute to our country, they should be allowed to stay and become citizens."

But what if, on balance, they cannot "contribute to our country"? What if Congress grants citizenship to millions of low-skilled immigrants who, no matter how diligent their work ethic or admirable their desire to assimilate, nevertheless end up costing taxpayers a bundle -- a bundle measured in the trillions?

In a recent study The Heritage Foundation's Robert Rector shed some much-needed light on how this latest wave of would-be citizens would interact with our welfare state. Rector studied the 4.5 million American households headed by immigrants without a high-school education and found:

• These households receive roughly $3 in immediate benefits and services ($30,160) for each $1 in taxes paid ($10,573);
   
• This $19,588 annual disparity occurs at each stage of the immigrants' life, moving from $14,654 for younger immigrants to $32,686 annually for immigrants age 65 and older;
   
• This fiscal drain exists despite evidence that low-skilled immigrants enter the workforce at twice the rate of households headed by low-skilled, native-born Americans and earn considerably more ($28,890 per household versus $17,581).

How can so much hard work in "tough and menial" jobs warrant so many government hand-outs?

To understand this, lawmakers must dispel one of the great myths of our welfare state. "It is a common misperception," Rector writes, "that the only individuals who are fiscally dependent are welfare recipients who perform little or no work, and that as long as an individual works regularly he must be a net tax producer." In fact, he continues, "the present welfare system is designed primarily to provide financial support to low-income working families." This support, moreover, extends well beyond traditional welfare to include health care, education, child care, nutrition assistance, housing, and retirement benefits such as Social Security and Medicare.

Indeed, according to a recent study by the Tax Foundation, 60 percent of Americans now live in households that "received more than one dollar of government spending for every dollar of taxes paid in 2004." Remarkably, the study's authors estimate the break point separating tax payers from tax consumers at approximately $65,000 per year, more than twice the average earnings of low-skilled immigrants.

If the "grand bargain" being debated in Congress grants citizenship to millions of low-skilled immigrants, lawmakers will have to pay these additional costs (Rector estimates the lifetime fiscal deficit for each low-skilled immigrant household to be $1.2 million) either through drastic reductions in welfare programs, Social Security and Medicare or equally drastic increases in our tax burden.

Americans instinctively understand the trade-offs -- and dislike them. In a recent poll, 70% of Americans said they believe illegal immigration weakens America because illegals use more public services than they pay for in taxes.

As one Bronx resident complained to The New York Times: "You build affordable housing and then put someone in it who hasn't paid their dues. Do Americans' tax dollars pay for us to be second?"

Mike Franc, who has held a number of positions on Capitol Hill, is vice president of Government Relations at The Heritage Foundation.