A win by Republican Scott Brown would give the GOP 41 votes in the Senate - enough to block final passage of a health-care overhaul. However, House Democrats could decide to swallow their objections that the Senate-passed version does too little for working families, and approve the Senate bill without changes. President Barack Obama could sign a final bill without another Senate vote.
Massachusetts is a classic blue state, where Democrats control both U.S. Senate seats and all 10 House districts - although 51 percent of voters aren't registered with either major party. A Brown win or even a narrow victory by Democrat Martha Coakley could discourage Democrats from running in competitive House or Senate races in November's elections amid a sagging economy and declining poll numbers for President Obama.
The Obama agenda
Obama has warned that much of his agenda in Congress, including a proposed fee on big banks, hinges on retaining the seat that Democrat Sen. Edward M. Kennedy held for nearly 47 years.