Please do not fall into the trap of assuming the special election in which Brown was elected was a low turnout one. Quite the opposite. The same number of people turned out in 2010 Senate special -- about 2,200,000 -- as did the 2010 general election -- again, about 2,200,000.
It's not a "trap," it's a solid point. It was high turnout for a special election--but that's a caveat. You know that turnout in 2012 will be much higher than it was in the 2010 general or special election. Brown would have won anyway, but Dem performance in cities like Lowell and Lawrence collapsed because people stayed home who only come out for Presidential elections.
Scott Brown got 1,168,107 votes and won in 2010.
John McCain got
only 60,000 votes fewer in 2008 and was absolutely trounced.
How many more people are going to vote for Brown who didn't vote for him in January 2010? Some, possibly... and maybe he won't lose too many who voted for him then because of his opponent or conditions at the time... but enough to get him to 50%? Tough. Can happen, but tough.