Not so much today, a time where roughly 20% staunch Republicans and staunch 20% Democrats battle for the 60% undecided (research this if you wish, the 20% on each side is a true fact).
I assume you are going by registered dems and reps. Many people are staunch supporters of either side but they do not register as such. However, your numbers are basically correct, the independents really decide every political election in the U.S.

as I do not consider it wise to sick to one single party for one's entirely life in a nation such as America, where politics tend to focus on individuals more than in an other nation - in most world states people vote stolidly by party, and so did the Americans some time ago.
Read Alexis de toqueville and James Bryce. they explain that exact statement more in depth.