TDS Co-Editor Ruy Teixeira has an article in The New Republic which brings clarity to the confused discussion about ‘swing voters.’ As Teixeira explains:

…There’s good reason to believe that the vast majority of Americans, including professional journalists and campaign operatives, wouldn’t recognize a typical swing voter if they met one.
Indeed, the application of the term “swing voter” deserves a lot more scrutiny than it generally earns. As it is, the term is thrown around carelessly to apply to large demographically or ideologically defined groups. The most common assumption, for example, is that swing voters are synonymous with political independents, but as I explained at length in a recent book review, that is an utterly fanciful notion.
Instead, the simplest and clearest way to think about it is on the level of the individual voter…The relevant criterion that needs to be fulfilled is persuadability. And that’s not a quality that’s exclusive only to those who are completely undecided, or who are only weakly committed to a candidate. Even those who are moderately committed can be persuaded to deepen their commitment. And the deepening of an existing affiliation with a candidate can be just as significant, both statistically and electorally speaking, as attracting mild commitment from someone who had previously been mildly committed to another candidate.

Teixeira walks his readers through some numerical exercises in “shifting probablility” in terms of commitment to candidates to illustrate the range of swing voters and adds,

Persuadability, then, is not logically restricted to voters in the center; it can potentially be far more broadly distributed. That is what William Mayer found in his analysis of swing voters based on National Election Study data. Swing voters are least likely to be found among strong partisans (12 percent of this group); more likely to be found among independent leaners (27 percent) and weak partisans (28 percent); and most likely to be found among pure independents (40 percent). But since pure independents are such a small group, they wind up being just 13 percent of all swing voters, actually less than the number of strong partisans among swingers (18 percent). Another 28 percent of swing voters are independent leaners, and the largest group, 42 percent, are weak partisans. Thus the overwhelming majority (70 percent) of swing voters are weak or independent leaning partisans–the kind of voters whose probability of support for “their” candidate is more usefully thought of as being movable from 70 to 80 percent than from 45 to 55 percent.

Nor do swing voters tend to be clustered in particular demographic groups, as Teixeira explains: “The idea that swingers are heavily concentrated in special groups like “soccer moms” or broader ones like the white working class or Hispanics is incorrect. In reality, swing voters are scattered throughout the social structure.”
But the broad distribution of swing voters does not mean they can’t be reached. “…Given the fact that the overwhelming majority of swing voters are partisans, the logical place for a campaign to start is by consolidating support among “their” swing voters–that is, by driving up their support rates among weak partisans and independent leaners.” He acknowledges that targeting swing voters with particular messages is very difficult, since “swing voters are everywhere.”
Teixeira suggests “setting support rate targets” for demographic groups,” then “figuring out how to hit each of the targets…however boring it may seem, that’s how you win elections.”