According to a Pew Center average of monthly polls, 33.7 percent of those surveyed identified themselves as independents in 2007. Those who identified themselves as Democrats were next in line, at 32.9 percent. Republicans have been losing voters since 2003, and finished last year with 25.4 percent of those polled — a historic low in the time that Pew has been surveying voters [ed. i.e. since 1944].
The decline relative to the Democrats pretty much coincides with the Iraq War during the Presidency of George W. Bush whose popularity is on a par with Nixon. The survey also reports that independents lean disproportionately towards the Democrats.