Pretty much everybody who follows domestic policy has understood, for a while, that expanding the State Children's Health Insurance Program (S-CHIP) was going to happen under President Obama--and that it would happen quickly.

Large, bipartisan majorities in both houses passed such a measure last year, as the program was set to expire. The idea was not merely to renew the program, which has brought health insurance to millions of children and their families, but also to relax its eligibility standards, since even many middle class families are struggling to find affordable coverage these days.

Despite the considerable Republican support, President Bush vetoed the measure, arguing that it would expand government-run health insurance and subsidize coverage for people who didn't really need the help. (I challenged those arguments here, for those who want to read the argument again.)

Efforts to override Bush's veto fell just short, so Congress contented itself with a small, one-year extension--in the hopes that Obama would win the eleciton. Once he did, enacting the expansion became a question of when, not whether.

But it's also turned into a question of how. After some initial talk of folding S-CHIP into the stimulus package, its chief advocates both in the Obama transition team and on Capitol Hill have decided to pass it as a separate measure. This would allow them to extend the programs for five or six years, as originally planned, and to do so with a dedicated funding stream--in the form of a higher cigarette tax.

Passing S-CHIP separately also promsies certain political virtues. It would give Obama a clean, easy win early in his administration--something that would, ideally, help Obama build some political capital.

But it's not a sure thing that it will be so clean and easy. The original S-CHIP measure, passed during the 1990s, barred even legal residents from claiming beneifts until they'd lived in the U.S. for five years. House Democrats, who are supposed to vote on the measure Wednesday, have decided to strike that measure. (Illegal immigrants would remain ineligible.)

I've heard Senate Democrats are leaning towards doing the same thing. But staffers I consulted said it wasn't a done deal, in part because they worry Republicans would use the change to attack the issue. Even if those attacks couldn't derail the S-CHIP expansion per se, it might stir up the kind of opposition that could stall their progress on other issues--not least among them, universal health insurance, an issue still on the docket for this year.

Whatever the strategic wisdom--and that's a judgment I'll leave to the actual strategists--including legal immigrants certainly makes sense on the merits. As this Kaiser Family Foundation primer notes, immigrant children are far more likely than native-born children to go without health insurance, not because their parents are less likely to be workng but because their parents are more likely to work in low-wage jobs that lack employer benefits.

As a result, these children are less likely to get preventative services and more likely to use emergency rooms. (See the chart below, from the Center on Budget and Policy Priorities.) They end up less healthy and, frequently, their families end up weighed down with huge medical bills. Meanwhile, the costs of subsdiziing what the families can't pay inevitably ripple through the rest of the health care system.

For more on the issue and the politics, see Robert Pear's write-up in today's Times

--Jonathan Cohn