Complex sentence: The association with HMOs begun in the 1980s was formalized under President Clinton in 1997 and in 2003, under President George W. Bush, a Medicare program for covering almost all drugs was passed (and went into effect in 2006).
Split into simpler sentences: The association with HMOs begun in the 1980s was formalized under President Clinton in 1997. In 2003, under President George W. Bush, a Medicare program for covering almost all drugs was passed (and went into effect in 2006).