Think the immigration debate boils down to whether the 10 million illegal immigrants already here deserve amnesty? Think again. The leading reform proposal in the Senate is Sens. Chuck Hagel and Mel Martinez's Comprehensive Immigration Reform Act (CIRA). If it becomes law, more than six times that figure will pour in - legally - over the next two decades.

The original CIRA would've allowed as many as 100 million people to legally immigrate to the United States over the next 20 years. We're talking about a seismic shift of unprecedented proportions.

Facing criticism, the Senate has amended the bill - which now, if enacted, would "only" allow around 66 million new immigrants. That still more than doubles the rate, from 1 million a year now to 2.5 million per year.

Current law would let 19 million legal immigrants enter the United States over the next 20 years; CIRA would add an extra 47 million.

This flow of new immigrants would dwarf the Great Migration of the late 19th and early 20th centuries. In that period, foreign-born persons made up no more than 15 percent of the U.S. population. In 1924, Congress passed a law greatly reducing future immigration. By 1970, foreign-born persons had fallen to 5 percent of the population.

In the last three decades, immigration has increased sharply. The foreign-born now make up about 12 percent of the population. But if CIRA were enacted, and 66 million new immigrants entered over the next 20 years, foreign-born persons would make up 22 percent of the U.S. population, far higher than at any point in U.S. history.

Why such explosive growth? Consider how the new law would work.

Under CIRA, immigrants could enter the country or attain lawful status within the country through eight channels. In each channel, immigrants would gain permanent residence and the right to become citizens:

Visas: About 950,000 persons now get permanent-residence visas every year. Over 20 years, the inflow of immigrants through this channel would be 19 million.

Amnesty: The bill would grant amnesty to roughly 10 million illegal immigrants now living in the U.S.

"Family chain" migration: Today's law limits the number of family-sponsored visas for secondary family members, such as adult brothers and sisters, to 480,000 per year - minus the number of visas given to immediate family members (spouses, minor children, and parents of U.S. citizens). CIRA would eliminate the deductions for immediate family members. The net increase in immigrants under this provision would be around 254,000 a year, or 5 million over 20 years.

Green cards: CIRA would increase the number of employment-based visas from 140,000 to 450,000 per year. For the first time, it would also exempt the spouses and children of workers from the cap. Total yearly immigration under this provision is likely to be 450,000 workers plus 540,000 family members. The net increase above current law over 20 years would be around 13.5 million persons.

Guest workers: The amended CIRA would let 200,000 people participate in the guest-worker program each year. Over 20 years, that works out to a total inflow of 4 million.

Spouses/children of guest workers: Guest workers could bring their spouses and children to the United States as permanent residents, adding another 4.8 million entrants over 20 years.

Spouses/children of illegal immigrants given amnesty: Illegals who got amnesty could bring their spouses and children into the country as legal permanent residents with the opportunity for full citizenship. The resulting number of spouses and children who'd enter the United States: at least 6 million.

Parents of naturalized citizens: CIRA would greatly increase the number of naturalized citizens - who would have an unlimited right to bring their parents into the country as legal permanent residents. The resulting number of parents who'd enter as permanent legal residents: around 3.5 million over 20 years.

In short, CIRA would bring profound change. It would transform the United States socially, economically and politically. Within two decades, the character of our country would differ dramatically from what exists today. Americans - and their elected officials - need to ask: Is that what we want?

Robert Rector is a senior research fellow domestic policy studies at the Heritage Foundation.