You might then ask, "If most agree that the status quo is broken, why not fix it?" There is a simple answer to this: Since advocates for either side can't get what they want, the current broken model works well enough.

Supporters of expanded immigration have tried but failed to increase legal immigration and to create paths to citizenship for most of the 11 to 12 million unauthorized. Since those options have not been attainable, the "broken" status quo is the next best option; it allows immigrant numbers to continue to grow, while giving millions of unauthorized migrants time to establish equities and roots in the United States, including via United States-born children.

Advocates for limits on immigration have also failed to achieve their goals. They want more effective enforcement in the workplace, but strongly oppose both increased legal immigration and another large-scale legalization. Hence for them, too, the broken status quo is the next best thing.

Meanwhile the two most interested groups - unauthorized migrants themselves and their employers - find the broken status quo satisfactory. It enables most unauthorized migrants to use false documents to circumvent the law and find United States employment at higher wages than at home. It enables businesses to hire them at wage and benefit levels lower than the market would otherwise require. This minority of employers might prefer legal rather than falsely documented employees, but not if they cost more.

As President Obama prepares to initiate a national conversation on comprehensive immigration reform in the coming months, he should keep in mind that any hope for an effective solution lies in the details.

As with the 1986 Immigration Reform and Control Act (IRCA), the last "comprehensive" reform, current comprehensive proposals include three key components:

1. A plan to give legal status and a path to citizenship for most of the unauthorized;

2. A promise of more effective workplace enforcement via credible identification to limit employers' ability to hire unauthorized workers;

3. A way to deal with the future influx of migrant workers.

Each of the three elements is complex and contentious.

For legalization: How many of the estimated 11 to 12 million unauthorized should be legalized? How many should be granted United States citizenship? Is the government able and willing to run such a program with both efficiency and credibility? What fees, proof of eligibility, and other requirements would be enforced?

For credible workplace identification: the tricky issues are cost, accuracy, penalties, liability, and privacy.

On the influx of migrant workers: the fracas centers on how much control employers get over temporary and permanent entries of low-wage workers.

The disputes over legalization and secure identification have not changed much since Congress stalemated on comprehensive immigration reform in 2006-07. On migrant workers, employer advocates know that the dismal job market makes it hard to pass the large temporary worker programs they want. Perhaps a commission - if employer advocates could shape its composition - could achieve the same thing administratively.

In the end what matters is not the concept, but the detail. One obvious example: What would prevent employer or other interest groups from finding ways to dominate a commission empowered to set numbers of migrant workers?

The United States immigration debate has been in a similar place before. It was the details that ensured that the comprehensive IRCA reforms of 1986 failed to reduce unauthorized migration.

In concept, IRCA prohibited the knowing employment of persons unlawfully in the United States. But the perverse details imposed by some of the same interest groups active today invited the pervasive fraud that made this concept unenforceable. IRCA required employers to examine workers' documents but prohibited them from checking the documents' validity.

The key to successful comprehensive immigration reform is to ensure that the details support, rather than detract from, its goals.

Obama's administration can start by requiring that any legalization plan be evaluated by the Government Accountability Office (GAO) before it is implemented, to ensure that the government can legalize those eligible but prevent the widespread fraud that occurred under IRCA.

Washington also should require the GAO to certify that an effective workplace enforcement system has been implemented before legalization kicks in. And any truly independent commission would need safeguards against being dominated by the most interested interest groups.

Change is clearly needed, but as the president sets out to restart dialogue on immigration reform, he must not allow interest groups to push through another round of "comprehensive" reform that promises one result with grand concepts but whose details take us in another direction.

Not paying close attention to these critical details will only further exacerbate public cynicism and disenchantment. And that would leave us right where we started, with a still-broken policy.

Philip Martin, professor of agricultural economics at the University of California, Davis, was a member of the US Commission on Agricultural Workers. Michael S. Teitelbaum, a demographer, was a member and vice chair of the US Commission on Immigration.

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