BOSTON — On the eve of the Senate election that could determine the fate of President Obama's agenda, Democrats scrambled to build a firebreak around the candidacy of Martha Coakley against the phenomenon of Scott Brown, the Republican Massachusetts state senator whose underdog campaign has surged as the vessel for national opposition to the Democrats' supermajority in the chamber.

Both teams, reinforced by senior political operatives from Washington and bevies of volunteers from beyond Massachusetts, made ardent appeals to supporters to brave freezing temperatures and vote in today's special election to fill the seat vacated by the death of Democratic Sen. Edward Kennedy.