What percentage of the US electorate is not committed to any political party?

What percentage of it switches allegiance?

This question is prompted by the current campaigns to chose Presidential candidates.In Britain, of course, we know who our prime ministerial candidates are and have no direct say in their selection.

Almost nobody in Britain declares themselves committed to any party, in that the membership of our political parties is tiny and, in recent times, there has been a very marked decline in the numbers of people who say things like 'I've always been a Conservative [party]voter and always will be' or who think in terms of being born a Tory [Conservative] or Labour voter by virtue of their family or their section of society or country being of that persuasion.

Our pollsters now have to confine themselves to defining Labour or Tory 'voters' as those who answer that they'd probably vote for that party in ordinary circumstances.