Bush`s share of the vote in 2004 by state correlated at the 0.98 level with his performance by state in 2000. What that means is that if you spent the last four years in a cave and just surfaced today and asked "What happened in the election?" you could be 96% (that`s 0.98 squared) accurate in guessing Bush`s share in each state with just three kinds of information: his 2000 performance, his new intercept (start Bush off 3.9 percentage points higher), and his new slope (for each 10 percentage points his 2000 share goes up per state, his 2004 share goes up 9.77 percentage points). For example, if he earned a 50% share in a particular state last time, you would expect him to earn 52.7 points this time (3.9 + (5 * 9.77).
Yet, holding the candidates` constant from one election to the next doesn`t necessarily mean stability of results at the state level. In sharp contrast, consider the 1952 and 1956 races between Dwight Eisenhower and Adlai Stevenson. Eisenhower`s overall share grew 2.3 points from 1952 to 1956, only a little less than Bush`s improvement from 2000 to 2004, but Ike`s share fell in 19 of 48 states.
The 1952 and 1956 races correlated only at the 0.78 level, meaning you could only be 61% accurate at plotting Eisenhower`s 1956 results knowing his 1952 results and Eisenhower`s intercept and slope for 1956. In other words, there was hugely more shifting at the state level between 1952 and 1956 than between 2000 and 2004. I do not know why this was.
I made up a table of stability of state-by-state voting over time. Below are the r-squareds for state-by-state correlations for the last eight elections. The similarity of the 2000 and 2004 elections by state were the highest in recent history.
For 1992 and 1996, I`ve laid out the correlations both with the GOP candidate by himself and with the GOP candidate plus Perot (i.e., the non-Democratic share of the vote). There seems to be an upward trend over time for elections to become more stable, although 1984 to 1988 was 88%, which is low only compared to 2000 to 2004 (96%). The 1992 and 1996 elections were somewhat perturbed by Perot and by Clinton, who had a certain amount of Southern appeal. [See table here.]