Washington ? Here’s an emerging line of attack you can expect to hear more of in the 2008 congressional campaigns – especially if you live near a vulnerable Democratic incumbent: “Democrats vote to give welfare benefits to illegal aliens.”

Or, even better: “Democrats vote to take benefits away from deserving senior citizens to pay for welfare for illegal aliens.”

Ugly? Absolutely. Devastating? So Republicans hope. True? No.

Bashing Democrats on immigration – accusing them of doing everything but carrying illegals’ luggage across the border – is a GOP mainstay. But the accusations that Republicans started to peddle last week reached a new low in dishonest nativism.

The first salvo involved the House version of the measure to extend the children’s health insurance plan, SCHIP.

“What we do is take, at the cost of seniors who get … choices of their own health-care plans, we take it away,” former speaker Dennis Hastert, R-Ill., claimed during the House debate. “We wipe it out, and we give it to people who are illegal aliens.”

“That bill, if it becomes law, would take $197 billion out of the Medicare trust fund, from our seniors, to give to illegal aliens,” charged Rep. Ron Lewis, R-Ky.

Leave aside the inflated numbers. Leave aside the scare talk about “our seniors.” (AARP, the seniors’ lobby, supports the bill.)

The provision at issue would repeal a 2006 requirement that everyone applying for Medicaid provide proof of citizenship – passports or original birth certificates. That might sound sensible, but it has been a cumbersome, expensive solution to a non-problem.

In 2005, when he was overseeing the Medicaid program for the Bush administration, Mark McClellan noted that an inspector general’s investigation did “not find particular problems regarding false allegations of citizenship, nor are we aware of any.”

Because many Medicaid applicants don’t have such papers easily at hand – they’re not the passport-carrying types – the requirement has resulted in tens of thousands of eligible children being denied coverage or kicked off the rolls and has cost states millions of dollars to administer.

The House provision makes the documentation requirement optional for states, which, after all, have an interest in seeing that their Medicaid dollars are spent properly. Adults on Medicaid would still have to prove citizenship, swear that their children are citizens and provide their children’s Social Security numbers. And states would have to conduct annual audits to ensure that no illegal immigrants are being covered.

Opponents point to Congressional Budget Office estimates that lifting the documentation requirement would raise costs $2 billion over 10 years. But, CBO Director Peter Orszag told me, that’s almost entirely because it would increase enrollment of eligible children.

Not that those inconvenient details matter much. “Target Dems Go the Extra Mile for Illegal Immigrants,” crowed the House Republicans’ campaign arm. It sent out individualized releases accusing vulnerable Democrats of voting “to give illegal immigrants government healthcare benefits.” You can see the 30-second ads coming.

But the debate over the health insurance bill looked tame next to the howls of outrage over a later vote on a proposal to change the agriculture spending bill to bar funds from being spent on illegal immigrants – specifically, for housing benefits or in hiring. That is, of course, prohibited under existing law, as California Republican Jerry Lewis acknowledged in making the motion.

Republicans had a legitimate beef with the way the vote was hustled to a premature close, giving Democrats a victory that might not have been theirs. But at a news conference the next day, Republicans took pains to emphasize that this was not simply about procedural mistreatment.

“The radical leadership of the Democrat Party reversed that vote in order to again give more government benefits to illegal immigrants,” said Jeb Hensarling of Texas, who chairs the conservative Republican Study Committee.

To the Democratic leadership, said Tennessee Republican Marsha Blackburn, “it is more important to let illegal immigrants be paid, fed and sheltered with U.S. taxpayer dollars than it is to let the voice of the American people be heard.”

Paid, fed and sheltered? Federal law already prohibits this. But this debate isn’t really about making good use of federal funds. It’s about using immigration as a weapon against at-risk Democrats – and assuming voters won’t bother to learn the truth.