Republican party identification, which had already been at fairly low levels, in fact appears to have slumped further since Inauguration Day, although the gains are being had not among Democrats but by voters who identify themselves as independent.
Several polls conducted within the last week have attracted attention for their notably low levels of self-reported Republican voters. In particular, ABC/WaPo reported the number of Republicans as 21 percent, CBS/NYT at 20 percent, NBC/WSJ also at 20 percent (not counting "leaners"), and Pew at 22 percent.
FOX, by contrast, which generally reports higher numbers of Republicans and Democrats but fewer independents, put the number of GOPers at 30 percent (although this nevertheless represents a decline from most of their recent polling). Rasmussen put the number of Republicans at 33.2 percent in March, essentially unchanged from recent months; they have yet to report their results from April.
The following chart combines the numbers from these six organizations since August 2008, while adding LOESS regression trendlines.
Per the LOESS curves, the number of Republicans has decreased by about 5 percent since Inauguration Day, from roughly 27 percent to 22 percent. The number of Democrats has also decreased slightly, however, from 38 percent to 35-36 percent. The gains have been made by independents, whose numbers have increased from 30 percent to about 36 percent, such that there are now roughly equal numbers of independents and Democrats.
The shifts in the number of Republicans and independents appears to be a somewhat recent phenomenon, dating not from Inauguration Day itself but rather from the past 50 days or so. My guess is that it is related to increasing -- if possibly unwarranted -- optimism about the economy, perhaps coupled with the GOP's lack of focus in articulating an agenda.