Second choice polls let us gauge breadth, depth of candidate support
Second-choice polls tell us a lot about the climate of opinion among a party's rank-and-file. They're a window on how voters will feel at the general election, after the dust of primaries has settled and a nominee is tapped.
The number of first choices a candidate receives is a good indication of his or her core support - that is, the number of voters preferring that candidate to all others. Second choices are good indicators of breadth of support - i.e. the number of voters who prefer a candidate when forced to consider viable ones. A poll that includes first and second choices lets us zero in on a likely winner - just like IRV balances core and broad support to zero in on a majority winner.
Hotline/Diageo's latest poll on 2008 hopefuls show the Dems settling on Hillary Clinton and a Republican party where much is in flux. Clinton leads first choices with 38% core support, followed by 14% for Kerry. While Kerry leads second choices at 22%, most of those respondents listed Clinton as their first choice. Under IRV, those ballots would transfer to Hillary. So would the 21% second choice votes she got from other candidates' supporters. Clearly, Clinton enjoys high levels of support - core and overall - within the party.
No Republican has an impressive plurality of first choice votes, telling us core support has not really coalesced anywhere - yet. McCain and Giuliani are close, but neither has serious second-choice support in the party. Frist and Gingrich share the same base of supporters; the second choices of each candidate's first-choice voters go to the other candidate.