Federalism Breeds Duopoly?

The authors of this
posit that creeping federalism turned American politics into the two-headed
monster it is today. If states had more power, they say, more parties might flourish,
as once they did. (Thanks, Dappled

[T]he United States has not always been so dominated by two parties. Third parties (sometimes even fourth, fifth and sixth parties) once competed successfully in congressional elections, winning significant portions of the popular vote and often gaining seats in Congress. This was true for most of the 19th century and even the early part of the 20th.