First, do no harm. That’s the rule in medicine; it should be the rule in expanding subsidized medical insurance.

Congressional Democrats and President Bush having a stare-down right now over the renewal of the State Children’s Health Insurance Program, better known as SCHIP.

SCHIP – at least originally – was intended to help states finance health coverage for poor children, those in families with incomes no more than twice the federal poverty level. It’s been successful, cutting the rate of uninsured children in these households by a third since 1998.

Now the House and Senate are pushing to expand the reach of SCHIP far beyond the poor.

In renewing the program, they want to move eligibility for sliding-scale subsidies into the middle class. This would insure more children – but would also lure parents into pulling their children out of existing private plans.

Many state leaders like it partly because it would send more federal money their way. Washington’s Legislature, for example, has already approved medical coverage for children in homes with incomes up to triple the federal poverty rate, about $62,000 a year for a family of four. Gov. Chris Gregoire and other Washington politicians strongly endorse SCHIP expansion, as do many hospitals and health care providers.

New York wants to move to quadruple the poverty rate, $82,600 for a family of four. Not surprisingly, Sen. Hillary Clinton of New York likes that level very much.

But the federal and state government shouldn’t be extending subsidized health insurance to families with comfortable incomes before children of lower incomes have been covered. SCHIP hasn’t come close to that goal.

Then there’s that pesky problem of “crowd out” – enticing families to drop private coverage for the a less-expensive government-subsidized plan.

The Democrats who want to expand SCHIP upward downplay the potential for this. But the Congressional Budget Office doesn’t. It estimates that, if the Democrats’ plans were enacted, one in three of the children enrolling in SCHIP by 2012 would be coming out of private plans.

In other words, the public would be financing insurance for children who would have it anyway.

Bush is hardly on the side of the angels in the SCHIP dispute. He isn’t proposing remotely enough money to provide enough coverage for poor children.

But he’s right in trying to preserve SCHIP as a poverty program. Any new money ought to be focused on children of low income before cutting more comfortable families into the deal.

This country may want to create another middle-class entitlement for health care. If so, it should be properly introduced at the front door, not sneaked in the back disguised as a poverty program.