The Senate has defied a veto threat from President Bush to approve an expansion of the child health insurance program, or S-CHIP. The 67-to-29 vote would be enough to override a presidential veto. But a showdown looms since the House failed to attract a two-thirds majority in passing its version of the measure. The bill would spend $35 billion to expand health insurance to more than four million children. The money would come through a boost in the federal tobacco tax. President Bush has promised a veto because he says it would increase the government’s role in healthcare.