Final answer:
The Republican support for drug benefit changes in 2003 was driven by control over the House, potential for Senate control, and backing from AARP, which saw the Republican plan as more viable. This strategic support helped pass legislation expanding Medicare's prescription drug coverage.
Step-by-step explanation:
Republicans supported drug benefit changes in 2003 primarily due to a combination of political strategy and support from influential groups. At the time, the Republican Party had control over the House and seemed poised to regain control of the Senate. Their position on Medicare drug benefit changes was backed by the American Association for Retired People (AARP), a powerful lobbying group advocating for seniors. AARP supported the Republican plan because it was more likely to be enacted than the Democratic proposal, which faced potential veto by Republican President George W. Bush. This support was instrumental in convincing moderate Democrats to back the bill and bolstered Republican success during the 2002 midterm elections. Furthermore, there was significant political pressure to provide a prescription drug benefit under Medicare; this expansion was compared with the lack of a strong advocacy group for the uninsured, which likely influenced legislative priorities.