FORMER SENATOR Jim DeMint (R-S.C.), now leader of the Heritage Foundation, knows that the nonpartisan Congressional Budget Office (CBO) is likely to judge that immigration reform — including eventual citizenship for millions of undocumented immigrants — will be a shot in the arm for the U.S. economy. After all, the CBO has done so with previous such legislation. That explains why Mr. DeMint, a bitter opponent of legalization, has launched a preemptive attack on the CBO — “puppets of the Congress,” he called the office the other day — and why Heritage has issued a study slamming amnesty for unauthorized immigrants as a drain on taxpayers.

The Heritage paper, chock-full of assumptions that most economists dispute, is a blatant attempt to twist the immigration debate. It concludes that newly legalized immigrants would cost $6.3 trillion more in benefits over their lifetime than they would pay in taxes. (That’s $5.3 trillion more than they would cost without legalization, the think tank said.) The study updates a similar one by Heritage in 2007, which pegged the fiscal cost of amnesty at that time at a mere $2.6 trillion.

There’s no question that granting the full range of government benefits to illegal immigrants — even if they become eligible for citizenship 13 or 15 years from now — will impose long-range fiscal costs. However, most economists say the costs of illegal immigration would be far outweighed by the benefits of legalization for overall economic activity,growth,business start-ups and labor market efficiency.

That’s not news for the construction industry in Arizona, where hostile state laws have driven away thousands of illegal immigrants and builders have scrambled to find scarce workers. It’s not news on farms from coast to coast, where more than half the labor force lacks documents and growers worry that their crops will go unpicked without a system to legalize unauthorized migrant workers.

Moreover, by ignoring the effects of legalization on the overall economy, Heritage failed to take into account the effects on federal revenue as workers emerge from the shadows to start businesses, travel without fear of arrest and deportation, earn higher wages and contribute to job creation.

The authors of the Heritage study acknowledge that the population of illegal immigrants, most of whom lack high school diplomas, would impose no greater burden on the budget than native-born Americans and legal immigrants with similar educational levels would. What Heritage really objects to is redistributive government programs, which one of the study’s authors termed America’s “cradle-to-grave welfare state.

Influential Republicans, including Rep. Paul Ryan of Wisconsin, Sen. Marco Rubio of Florida and former Mississippi governor Haley Barbour, rolled their eyes at the Heritage report with varying degrees of politeness. In a letter last month to Heritage, Mr. Rubio, who has risked his political future and putative presidential aspirations to champion immigration reform, noted that the real impact of legalization and citizenship must take into account “both its baseline costs and its impact on growth.” Mr. Barbour, who doesn’t appear to be running for anything, could afford to be more blunt. “It’s a political document,” he wrote on Twitter. “It’s not a serious analysis.”

That’s true, as the CBO is likely to make clear when it publishes what is certain to be its more dispassionate, and less political, assessment of the proposed legislation. In the meantime, lawmakers should bear in mind that waves of previous immigrants — Irish, Italians, Jews, Germans and others — have been greeted by prophesies of doom. Still, the United States thrived — with the help of those same poorly paid, roughly educated newcomers whose integration triggered such derision.