Looking at counties with more than half a million people (there were 63 of them according to 2000 Census figures), I calculated the fraction of votes for Obama in 2008 and for Nader in 2000. The rankings are presented below.
The scatter plot below provides a clearer picture of how these counties differ along these two measures of political ideology.
Figure: Scatter plot of Counties, by Obama and Nader Vote Share Rank
This figure highlights how difficult it is to measure ideology. In 2000, only three counties had a higher Nader share than Salt Lake, Utah, however, only three counties had a lower Obama vote share than Salt Lake. In other words, Salt Lake is liberal according to one measure, but conservative according to another. Travis, Texas (home to Austin) is similar in this regard. No county had a higher Nader share in 2000, though it did not have a particularly high Obama vote share in 2008, relative to the other counties.
On the other end, Philadelphia, Prince George's, and the Bronx ranked high in Obama shares but low in Nader vote shares.
Meanwhile, New York (Manhattan), San Francisco and Alameda (Oakland) ranked high in both Obama and Nadar vote shares, while Tarrant, Texas (Fort Worth) ranked low in both Obama and Nader vote shares.
The source of the voting data is Professor John Mackenzie’s wonderful collection of data files; you can browse through his collection by clicking here.
Table: U.S. Counties with Population > 500,000, by Obama and Nader vote share