A coalition of 59,000 nuns released a letter yesterday calling on Congress to approve the overhaul, defying the US Conference of Catholic Bishops, which opposes the measure. The Catholic Health Association, which represents 1,200 Catholic hospitals, has endorsed the package, as have Catholics United and Catholic groups promoting social justice. That split mirrors a division among some antiabortion US representatives. In preparing to cast perhaps one of the most important votes on a domestic issue in their careers, they are wrestling with questions that strike at the core of their beliefs and that threaten to embolden voters in November. Ardently antiabortion Representative Dale Kildee, a Michigan Democrat who once studied in a Catholic seminary, said yesterday he will vote for the package despite language that some believe is not strict enough in ensuring that no federal funds are used for abortions.