The humanitarian crisis of tens of thousands of unaccompanied Central American children streaming across the U.S.-Mexican border not only poses painful immediate policy choices, but further confounds our already polarized national dialogue on long-term immigration policy.

Regarding this particular influx, it can be blamed on a 1998 law aimed at international child trafficking – passed with broad bipartisan support and signed into law by President George W. Bush – that bars the immediate deportation of any unaccompanied minors from non-contiguous (i.e. other than Canada or Mexico) countries, and allows them to stay in the U.S. – legally and with few restrictions – until they have a deportation hearing, something that can take anywhere from two to four years.

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Unquestionably, back in Guatemala, Honduras and El Salvador, people have become aware of this law and its operation, so their gamble in dispatching children on this perilous journey – that if they can make it to the American border, they will be able to stay in the U.S. (perhaps indefinitely) – is not entirely unjustified.

What bearing does this crisis have on the larger national immigration policy discourse? It’s all about enforcing border security.

Comprehensive immigration reform rests on three policy issues:

1. Determining the long-term status of the 11 million or so illegal immigrants now in the U.S.

2. Preventing prospective illegal immigrants from entering the U.S.

3. Redesigning the quotas for admitting prospective legal immigrants.

In theory, most Americans actually agree on the broad outlines of policy in all three areas: rigorously secure the border; institute a process that enables law-abiding illegal immigrants – especially young ones – to establish legal residency (with or without full citizenship); and increase legal admissions quotas for prospective immigrants in needed occupations.

That it should be politically possible to craft a “grand bargain” on these terms can be seen in polling data showing this policy mix supported not only by Democrats, but a majority of Independents and a solid plurality of Republicans. It also happens to form the essence of the Senate immigration reform bill passed in 2013.

What is keeping this shared policy consensus from making it across the legislative finish line (it is hopelessly bottled up in the House) is widespread mistrust regarding the enforcement piece.

Conservative and independent voters just don’t believe that this administration -- or perhaps any administration and congress under the sway of immigration “soft-liners” -- will ever have the will or resources to make good on the curtailment of illegal entry.

Admittedly, sealing the border is no easy task. However, the key border security provisions of the Senate bill, if enacted – and consistently enforced – should go a long way to achieving this.

Besides beefing up personnel and barriers at actual border crossings, there are stringent new procedures for “interior enforcement:” requiring all employers to screen all prospective employees electronically against the national Social Security data base (most illegal workers are hired with fraudulent Social Security ID.

With that being detectable, employers would be taking much greater risks in employing them, removing the main rationale for entering the U.S. illegally) and making sure that all those who enter the country with visitor visas leave when they are supposed to by electronically linking visitor arrivals and departures and tracking down those not exiting on time (a majority of illegal immigrants arrive legally as visitors, not by sneaking across the border).

If the president and congressional liberals want to see comprehensive immigration reform enacted in their lifetime, they must prove that they are deadly serious about border security.

That is where the current border crisis comes in. How they propose to deal with the surge of Central American child migrants will serve as an important test of their sincerity on this issue.

Proponents of immigration reform must join congressional hard-liners in reversing the provision barring the immediate deportation of “unaccompanied alien children from non-contiguous countries,” and support the president’s proposal to make sufficient court resources available to quickly adjudicate the status of those already here.

Beyond that, liberals should agree to undertake comprehensive immigration reform in stages, with border security measures adopted first – but with a binding commitment from conservative legislators that once there is evidence that the pace of illegal immigration has been substantially curtailed, they will go along with enacting the other two pieces of the reform package: a pathway to legalization and a more economically rational quota system.