Party affiliation, as reflected in polls, is weirdly volatile. While experience suggests that most people change their party affiliation only rarely if at all, what respondents tell pollsters tends to bounce around. Currently, Gallup finds that slightly more Americans identify as Republicans than Democrats. Click to enlarge:
At the link, you can see the numbers going back to 2004. For what it is worth, in October 2016 the Democrats held a five-point lead. On the negative side, when independents were asked which way they lean, they said Democrat by a three-point margin.
Still, the recent shift toward Republican identification is interesting. Maybe some voters, when they focus more closely on what Democrats are doing, don’t like what they see. Maybe the Democrats’ violent wing, Antifa and BLM, has tarred the party.
As I understand how conventional polling works, pollsters weight their raw findings to achieve demographic and political balance. Thus, if they happen to oversample men, they will adjust the numbers to reflect the gender breakdown of the electorate. Similarly, I believe they adjust the raw numbers to reflect their assumptions about the breakdown among Republicans, Democrats and independents among likely voters. I may be wrong about this, but my understanding is that an incorrect assumption about party identification will throw off poll numbers as reported. Those who understand this better than I do can contribute in the comments.
In any event, it is extraordinary that, despite non-stop propaganda from the press, academia and Hollywood, at least as many Americans currently consider themselves Republicans as Democrats.