As expected, President Bush vetoed legislation Wednesday that would expand a children’s health insurance program, drawing immediate fire from congressional Democrats and setting up an override showdown that could be politically perilous for some Republicans.

The bill Bush vetoed (HR 976) would expand the State Children’s Health Insurance Program, or SCHIP, by $35 billion over the next five years, to $60 billion. That would be enough, Democrats say, to provide health coverage to 10 million children whose families are low-income but not poor enough to qualify for Medicaid — about 4 million more than the program now covers.

Expanding the program has been a major priority for congressional Democrats, who have talked of legislative measures to assist middle-class families stressed by stagnant wages, a deteriorating housing market and rapidly growing health care costs.

Bush had threatened to veto the bill for months, arguing it would cost too much and would encourage some parents to drop private family coverage. About 2 million people who have health insurance or could obtain it would instead join SCHIP, according to the Congressional Budget Office.

“If this bill were enacted, one out of every three children moving onto government coverage would be moving from private coverage,” Bush said in a statement accompanying the veto. “The bill also does not fully fund all its new spending, obscuring the true cost of the bill’s expansion of SCHIP, and it raises taxes on working Americans.”

Democrats expressed disappointment with the president’s decision.

“It is a shame that the president did not step away from his threats against this children’s health bill,” said Senate Finance Chairman Max Baucus, D-Mont. “Let us be clear about what this veto means. The president is saying that millions of low-income, uninsured American children must continue to live with no health coverage while he presses his ideological concerns.”