Senators like to think of themselves as members of "the world's greatest deliberative body" -- where lawmakers subject every aspect of major legislation to withering scrutiny before allowing it to come for a final vote. However, particularly intense political pressure often turns important bills into legislative freight trains, where analysis and reflection give way to rash judgments.

Describing the legislative frenzy known as "The Hundred Days," during which President Franklin D. Roosevelt won congressional approval for his historic New Deal agenda, one senator observed ruefully that Congress "counted deuces as aces, reasoned from non-existent premises and, at times, we seemed to accept chimeras, fantasies and exploded social and economic theories as our authentic guides."

Little has changed. This week the Senate may gives its approval to another historic legislative proposal -- a massive rewrite of our immigration laws. The 614-page bill has received neither a public hearing nor any other form of scrutiny from the Senate Judiciary Committee. According to Sen. Jeff Sessions (R.-Ala.), a committee member, his colleagues are "blissfully ignorant of the scope and impact of the legislation."

Remarkably, until last week, no one had bothered to conduct a serious section-by-section review of the bill to calculate just how many additional legal immigrants would be allowed to settle in the U.S. and embark on the path to citizenship. Nor did anyone examine the potential fiscal effects of this complex web of highly technical provisions.

To be fair, over the last two years Senate Judiciary Committee Chairman, Arlen Specter (R.-Pa.), held many hearings on immigration policy. Fifty expert witnesses appeared, addressing issues such as the best way to end violence at the border, reduce immigration-related litigation, secure the cooperation of other nations, safeguard our national security, reform laws governing the deportation of illegals, and so on.

Only two hearings examined the economic impact of immigration. None examined the possible fiscal consequences or the potential scale of future immigration, and what these consequences might mean for our country.

So, this week senators embarked on one of the more intricate legislative dances in recent years without knowing some of the most basic steps. That they are likely to stumble goes without saying.

Luckily, my colleague Robert Rector read the bill, asked those basic questions, and ran the numbers. The Senate immigration plan, he found, essentially creates an entitlement to citizenship. Tens of millions of foreign citizens would qualify for the unilateral right to come to America and apply for citizenship. (The precise number, as of this writing, appears to have dropped considerably from Rector's original estimate of 103 million to a "mere" 66 million over 20 years, thanks to the Senate's adoption of an amendment by Sen. Jeff Bingaman (D.-N.M.)) Because many would likely be low-skilled high school dropouts who would work in low-wage jobs, they can be expected to consume more in social services than they pay in taxes. Rector characterizes this possibility as "the largest expansion of the welfare state in 35 years."

Rector shows that immigrant households are already 50 percent more likely than native-born households to use welfare. Immigrants without a high-school degree are two-and-a-half times more likely to use welfare. That means billions in additional spending on programs such as Food Stamps, Medicaid and the Earned Income Tax Credit. This is not your grandfather's welfare state.

The official congressional scorekeeper, the Congressional Budget Office, reviewed the same provisions and arrived at a very different conclusion. The immigration plan, it estimates, would attract only 7.8 million additional immigrants over the next 10 years. If they were commodity traders, CBO's immigration experts would surely sell the value of American citizenship short. How else to explain its assumption that only a miniscule proportion of those who would be eligible to come to America would actually do so?

Those who regard the prospect of American citizenship as one of life's most valuable commodities -- especially to citizens languishing in impoverished countries where annual incomes are but a fraction of ours -- believe that virtually all of those eligible to become Americans would jump at the opportunity to do so. In fact, recent polls in Nicaragua and Peru belie CBO's assumption. Those polls found that 60 percent of Nicaraguans and 70 percent of Peruvians would move to the U.S. if they could.

This is precisely the sort of debate that the "world's greatest deliberative body" should have had long before now.

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