After some calculating from PA Department of State online files, I figured out the Chesco voter registration by party since 1998:
In the line chart, the red line shows Republican registration, blue Dem, and yellow Other. The square dots show alternating general and primary elections.
From November, 1998, through November, 2007, there was a slow rise in the Dem %, with a slight upward bump for the general election of 2004 (Kerry v. Bush). Also in 2004, there was a bump up for Others and down for R’s, suggesting perhaps some anti-Bush movement.
Dem registration continued its slow rise (perhaps this rising baseline is due to demographic factors) and then took a large jump with the primary (Obama v. Clinton) and general (Obama v. McCain) elections of 2008. Both R’s and Others contributed to that D jump.
Since then, the D and R numbers and (next chart) the D to R ratio year by year have been quite stable. Why? Because voters are still waiting to see how the D v. R, Obama v. Boehner, US Senate v. US House, and R v. Tea Party dynamics play out? In any case, perhaps this is a sign that Dems can’t count much on continuing demographic changes to make their case, or, at least, that they will have to go to great efforts to improve their numbers till the next presidential election–assuming they have a popular candidate then.
In all, over these 14 years, D registrations rose from 28% to 38% and Others from 15% to 17% (with a slight temporary slip in 2008), while R registrations have declined pretty steadily from 57% to 45%.
Finally, here are the underlying numbers. The 3rd parties (Green, Constitution, Libertarian, Reform) listed in the state data vary with year; all are here counted as Other.