With the White House at stake this year, the latest round of voter registration brought a fairly major shift in the party demographics in New York State: Several former Republican strongholds have seen increases in the ranks of Democrats, including in two formerly-unlikely areas.

But now Republicans are falling behind even in places like Long Island, where Nassau County — once home to the most formidable Republican machine in the country — reported this week, for the first time in its history, having more registered Democrats than Republicans. Likewise, in Dutchess County, which runs from the Hudson east to the Connecticut line, Democrats overtook Republicans in enrollment on Wednesday, according to election officials there….

For symbolic and practical reasons, the turnover in Nassau County is perhaps the most stark. Over the years, the Republican organization there has produced many leading Republican politicians, among them former United States Senator Alfonse M. D’Amato and Dean G. Skelos, the Senate majority leader.

“Nassau has been around for 110 years,” said Jay Jacobs, the Democratic county chairman. “For 110 years it’s been Republican. This is a big deal.”

Of course, just like the rest of the country, party affiliation is extremely flexible. The GOP will still get votes in the right situations, locally and state-wide.

But to me, this reflects the larger demographic shift that’s been happening in the so-called “first suburbs” of Nassau and Westchester. These post-World War II bedroom communities are losing their former affluent residents to the further-flung exurbs, and thus are morphing to resemble their adjoining urban areas, i.e. are getting more diverse and less rich. So naturally, they’re gravitating more toward the Democratic Party. I expect the same party shifts are happening to other first suburbs around the country.