Out here in Minnesota--whose gift to the world was the HMO system-- there have been quite a few interesting folks involved with the single- payer question. Early in his first term, Paul Wellstone was seriously interested in pushing single-payer initiatives--not that he thought it was practical to attempt it nationally at that point; his notion was to pursue measures that would make it easier for states to do single-payer experiments and thus try for a groundswell that way. But he was seduced by Hillary Clinton during her '93 March to the Sea, and he's never made a noise about it since, to my knowledge. (By the way-- why does everyone persist in claiming that the Clinton plan failed, just because it wasn't passed into law? It's quite obvious that the administration's move in the direction of HMOs touched off a merger mania that made her plan--minus some of its rube goldberg convolutions--into reality.)
I share Carrol's pessimism, but there are two grounds for some hope: the hideous and publicly reviled performance of the HMOs themselves, which has laid bare the workings of for-profit medicine like never before; and the concentration of medical/industrial resources touched off by the HMO craze, which--presuming a shift in the political tides-- could actually help ease the transition to a centralized, single-payer system. Remember, capital is not entirely unified against single-payer systems, and could become even less so; the most oft-cited impetus for the Clinton initiative was Lee Iacocca's complaint that he couldn't compete with the Japanese when he had ~$700 in health insurance costs built into each car, and they had ~$200.
On the question of public support, Wellstone told me of polls he was using (not sure where they originated) back in '92 showing that 7 of 10 Americans favored a Canadian-style system. Of course that eroded dramatically in the face of the '93 propaganda blitz, but the drift of public sentiment is clearly in that direction.