Does Massachusetts even have a runoff law?
As I recall, Vermont has one of sorts — strictly speaking if the leading candidate receives less than 50%, then the decision goes to the state legislature. It happened a few years ago, with Douglas (R) getting 49% or so and the Democrat getting maybe 42%. The Democrat waived his right to the decision going to the legislature, even though Democrats controlled it at the time. It didn’t matter much; Douglas was a RINO as I recall.
Connecticut does not — Rowland (R) won with about 38% one year, with the rest going to the Democrat and then to the Connecticut Party (a Lowell Weicker-inspired party) candidate.
Hmm...it appears that MA does have runoffs for primaries; I do find any provision for a runoff in the general election. Indeed, it looks like Romney got only 49.77% of the total vote, even if it was 51% of the enrolled party vote.
Republican Mitt Romney 1,091,988 49.77
Democratic Shannon O’Brien 985,981 44.94
Green-Rainbow Jill Stein 76,530 3.49
Libertarian Carla Howell 23,044 1.05
Independent Barbara C. Johnson 15,335 0.70
Write-in All others 1,301 0.06