The Senate's reasoning is that illegal immigrants who have been in the US less than two years haven't yet developed deep roots here - certainly not deep enough to be on the path to American citizenship.
So they're the ones who must pack up and leave, under the recently approved Senate version of immigration reform. All 2 million of them.
But the particulars of how that exodus would happen - including who would be responsible for enforcing it - remain hazy. Moreover, the bill requires a second category of immigrants - almost 3 million people who've been in the US two to five years - to return to a border to apply for a work visa, a process that critics say threatens to bog down an already overtaxed system.
Of course, it's uncertain if the Senate's version of reform will be the legislation that prevails, especially with House leaders firmly opposed to a path to citizenship for any of the estimated 12 million undocumented immigrants in the United States.
Either way, the challenge of enforcing a mass deportation of some 2 million people, and of sorting the remaining 10 million-plus into two distinct categories, is at the very least daunting and, at most, unfeasible, say those tracking Congress's efforts.
"Provisions that unnecessarily complicate what's already going to be a difficult and overwhelming process, to the extent that they can't be implemented, will undermine the success of the entire bill," says Deborah Meyers, a senior policy analyst with the Migration Policy Institute, who supports the sort of comprehensive approach the Senate bill has tried to take. "To be effective, certain changes need to be made [in the reform legislation], and they need to keep in mind the workability - not just what can get passed politically, but what can work when you implement it."
Under the Senate plan, immigrants who have been in the US longer than five years could register and stay and eventually apply for permanent legal status, so long as they fulfill certain requirements, including paying owed taxes and fines and learning English and US civics. Those in the middle category would be allowed to reenter the country on a work visa, and could eventually work toward a green card.
House leaders, however, are reportedly hardening their opposition to the Senate bill, raising the possibility of an impasse.
For now, Ms. Meyers and others question how burdensome the proposal might be on immigration agents, who would need to check and verify proofs of residency, and on border agents, who would have to process in a short time the nearly 3 million immigrants who fall into the two- to five-year category. The potential for fraud is a worry, and many doubt that immigrants who've been here fewer than two years would leave voluntarily.
But given a bill that survived the Senate largely intact against long odds, many supporters say such criticisms are quibbles about what is, in the end, the comprehensive approach they were seeking. And, they say, the system could work better than some anticipate.
"It's going to take a greater level of incentive to get people to come out of the shadows who have been here a long time, and less amount of incentive for people who have been here less time," says a Senate aide familiar with the bill, who required anonymity because he is not authorized to speak publicly about it. "It makes sense to treat people differently."
The aide dismisses the notion that returning to the border is merely a symbolic gesture. That's because the two- to five-year group would ultimately be considered temporary workers - even if they can eventually get on the path to green cards.
The Senate bill was passed with the expectation that the fees paid by immigrants who have been here more than two years would cover the costs of registering and and processing them. And it tries to head off the possibility of fraud by increasing penalties - including criminal prosecution, which would bar those guilty of using fraudulent documents from ever being considered for legal residency.
The bill also significantly beefs up border security - an aspect President Bush focused on during his visit to New Mexico Tuesday - by authorizing 370 miles of new fencing along the US-Mexico border and adding 14,000 more Border Patrol agents by 2011, more than doubling the current force.
Many who favor the general approach of the Senate bill - with its central provisions to boost enforcement, create a path to legal status for a majority of current illegal immigrants, and implement an expandable guest-worker program - say that so long as that architecture is in place, they can live with an unwieldy three-tiered system.
"If I were the [Immigration and Customs Enforcement agency,] I would be very strongly pushing to streamline the system," says Cecilia Muñoz, vice president of the office of research, advocacy, and legislation for the National Council of La Raza. Still, she's happy overall with the bill. Unlike some critics, she believes most of the 3 million people in the middle group would voluntarily return to the border to be processed - provided they are fully informed about the new system and can apply for legalization anonymously at first so that deportation is less of a worry.
Already, she says, immigrants misunderstand some aspects of the plan. Some believe, wrongly, that a parking ticket can keep them from being eligible for legal residency, or they don't know that they can apply for legal status for a spouse and minor children who haven't been here five years.
And the roughly 2 million who have been here less than two years?
"At a minimum, they would have access to the new temporary-worker program," says Ms. Muñoz. "I don't think anybody expects that the size of the undocumented population when this is done will be zero. But there's a big difference between 2 million and 12 million."
Muñoz is more optimistic than some other immigrant-rights advocates, who consider the three-tiered system so problematic as to ruin the whole bill.
"The truth of the matter is that the immigration system is broken, and the way this is presented right now is not fixing the problem," says Juan Carlos Ruiz, general coordinator for the National Capital Immigration Coalition, which represents immigrants in the Washington area.
Mr. Ruiz says he's spoken with many undocumented immigrants who are afraid to return to the border now that Mr. Bush is sending National Guard troops there. "We're asking them to leave their jobs, do a trip down there, and apply for a permit that they might not even get.... How can we be trusting this?"
The Senate's Immigration bill divides the nation's 11.5 to 12 million illegal immigrant population into three distinct groups. Here's what each is eligible for:
Arrived after Jan. 7, 2004
• Must leave the country, but could later apply for a temporary-worker visa from their home country (and might be able to waive some three- or 10-year bars on entry).
• Numbers: Nearly 2 million people.
In the US between two and five years:
• Eligible for "deferred mandatory departure" program if they were continuously employed for that time, pass all background checks, and pay a $1,000 fine and application fees. Would need to leave the US and apply to reenter through a port of entry, where he or she would receive a work visa for up to three years. Would be eligible for permanent residency after existing backlogs are cleared.
• Numbers: Roughly 2.8 million
In the US more than five years:
• Eligible for the earned legalization program, provided they worked a minimum of three years out of that five-year period, pass background checks, pay all owed federal and state income taxes, meet requirements for English and US civics, register for military selective service, and pay $3,250 in fines and fees. Could be awarded a green card once existing visa backlogs are cleared. Can also apply for legalization for immediate family members who don't fall into this category.
• Numbers: Roughly 6.7 million
Note: Exceptions and waivers are possible for some of the requirements.