Independents are unquestionably the key constituency of this election cycle, party because Obama peeled off so many "soft" voters in 2008 who are expressing some buyer's remorse and partly because voters look for new answers in tough times. A New York Times/CBS News poll, for example, found that independents have soured on Obama by 20 points in the last two years.
But independence is a gray scale, and as Truman's quote illustrates, some self-fashioned independents are more reluctant than others to deviate from habit.
The map below from Robert David Sullivan shows the degree to which different regions are likely to vote for candidates other than the Republican and Democratic standard-bearers in presidential elections. It doesn't show voters' willingness to flip-flop between parties (this map does), but it does shows which regions lack strong party allegiances.
Voters in New England, the Lutheran Belt, the northern Rocky Mountain States, the Deseret States, and the Pacific Northwest tend to vote for the "other" candidates the most often. This may not bode well for Harry Reid or Michael Bennet. Nor is it a good sign for Democratic House incumbents Earl Pomeroy (ND), Walt Minnick (ID), Kurt Schrader (OR), or John Salazar (CO).
The areas that veer from their party allegiance least often are the Black Belt in the South, the Tex-Mex border counties, and to a lesser extent, the entire Baptist Belt.
As for Truman's hometown of Independence, MO, it is somewhere in the middle. Of course, Truman was known to stubbornly stick with his views despite approval ratings in the cellar. I don't expect voters in the 2010 midterms to be as set in their ways.