Pollster Scott Rasmussen weights his results by party identification, but also readjusts party identification monthly so he won't miss underlying changes in the political balance. Party identification tends to change slowly, and he measures it using tenths of a percentage. My recollection is that party identification remained fairly steady in the 1995-2005 period; Rasmussen's numbers suggest it has changed more since mid-2005. Let's look at it quarter by quarter, rounding off to full percentages:
The Democrats' edge in the 2004 election quarter was 39 percent to 37 percent, not much different from the 37 percent to 37 percent party identification in the 2004 exit poll. It's one indication, and there were others, that Republican turnout efforts were more successful than Democratic turnout efforts. Party ID for both parties seems to have declined slightly after the election quarter, as the intensity of partisan feeling died down. Republicans continue to trail Democrats by about the same 1 percent to 3 percent up through March 2006.
Then in the third quarter, with the increased intensity of the fighting in Iraq, the Democratic edge grows. Republican Party identification sinks from 34 percent to about 31 or 32 percent, while Democratic Party identification rises to about 37 or 38 percent. Rasmussen spotlights the June 2007 result, which I've shown separately, as perhaps a turning point. What we may be seeing here is some discontent with the performance of the Democratic Congress at a time when the job ratings of Congress as a whole (and of both parties in Congress) is very low. But Republicans are still in worse shape than they were in October 2004-March 2006, while Democrats are in about the same shape they were in April 2005-March 2006.
Republicans may have an opportunity to raise Republican Party ID back up to 2004 levels. Rasmussen notes that Republican Party ID increased during a time when most Republicans in Congress were fighting the Bush-supported immigration bill. There may be some more rallying to Republicans with the commutation of Scooter Libby's sentence and with the forthcoming battles between Bush and the Democratic Congress over spending. But it's too soon to say that Republicans have definitely turned a corner.