If President Bush had a history of opposing big government and big spending, his choice of a child health insurance bill for only the fourth veto of his presidency might be more understandable.

But he has calmly presided over the largest increase in spending and the creation of the largest government entitlement — prescription drugs — since the Great Society. It is probably not far off to say his abrupt conversion to limited government and fiscal prudence has everything to do with the Democrats now being in charge of Congress.

The veto, however, has put his fellow Republicans, who voted for the measure in large numbers, in a tough spot politically. The measure, which subsidizes health insurance for low-income children, is popular with the states, the medical community and the families of the children. This bill would have added about 4 million children to the over 6 million already insured.

Bush, who had locked himself into a veto commitment, seemed to sense the political difficulty he was creating. He signed the veto privately, with no fanfare, and then quickly left town for a Chamber of Commerce speech.

Nonetheless, the Democrats were ready with ads targeting Republican opponents of the bill in vulnerable districts. The name alone — State Children’s Health Insurance Program, “S-chip” in Washington parlance — suggests how difficult a “no” vote is.

The bill has problems. It is expensive, $35 billion over five years; the use of a cigarette tax to finance it is questionable; and it may indeed cover some families who could afford private insurance.

But it is not, as some overheated opponents charge, socialized medicine or anything like it. Some Republicans charge that SCHIP is the first step toward “HillaryCare,” but even if that were in the back of the Democratic candidate’s mind, it would be beside the point.

As for competing with private insurance, as Bush charges, the president greatly overestimates the availability and affordability of purely private insurance.

Bush has indicated that he is open to some kind of accommodation. If Congress can reach some kind of reasonable compromise, fine. If not, it should override his veto.