Obviously a Scott Brown victory would be about as big a political upset as there's ever been, but it wouldn't be that much more amazing than some of the special elections Democrats have won on the House side in the last six years.
In the 2004 and 2008 election cycles Democrats won four House seats in districts that ended up going for the Republican Presidential nominee by 16 points or more:
-Stephanie Herseth replaced Bill Janklow in South Dakota before her district voted 60-38 for George W. Bush in the fall.
-Ben Chandler won Ernie Fletcher's House seat in Kentucky in 2004 as well before his district supported Bush's reelection by a 58-41 margin.
-In 2008 Don Cazayoux won Richard Baker's old seat in Louisiana before losing it in the general election when his district went for John McCain by a 57-41 margin.
-And perhaps most improbably Travis Childers took Roger Wicker's old district in Mississippi both in a special and in the general election even as McCain took it 62-38.
Massachusetts voted even more Democratic in 2008 than any of those districts voted Republican, giving Barack Obama a 62-36 victory. But all of those Democratic victories are a reminder that crazy things can happen in special elections, especially when there's such a large enthusiasm gap.