U.S. Politics/US Primaries
I'm from England and I'm trying to grasp what's going on in the USA at the moment.
As I understand it the voting we're seeing going on now and over the next few weeks is to choose a candidate for the Republicans and Democrats to stand for election as President in November, so
1. To vote at this stage in the Primaries (is that what they're called?) you have to be a registered member of either party? "Ordinary" folk can't vote at this point unless you're a party member?
2. If a candidate wins a majority of States he/she is the one to stand in November?
3. How do voters in Washington DC or Puerto Rico for instance take part in this process?
4. Does a sitting President who wants to carry on have to go through this process (Obama in 2012 or Bush in 2004 for instance) or are they their parties' automatic choice?
It's all so different over here.
I'll be glad to try to clarify what is admittedly a rather muddled - and very, very protracted - process. The numbers below correspond to the numbers in your question.
1) Most states have primaries, but some have caucuses. The difference is that, in a primary, a person goes to the polling place, casts a ballot, then leaves. In a caucus (generally speaking), voters congregate in a big room and vote en masse until the majority have selected a candidate. Now, whether a registered member of a party can vote in a primary or caucus varies state-by-state. In New Hampshire, non-party members can vote in either party's primary. But in Pennsylvania, only registered party members can vote in the party primaries. So, whether an independent can vote in the Democratic or Republican primary (or caucus) is dependent on where that voter lives.
2) The party nominee is determined not by the number of states won during the primary process but by the number of delegates accrued in that process. A candidate's success in each state determined how many delegates that candidate receives from that state. Each state has a different number of delegates which - for the sake of simplicity - is comparable to the population of that state. So that California has more delegates than Alaska, for example. Each candidates goal is to do well enough in each primary/caucus to be awarded a majority of the delegates who will be attending the party conventions in the summer. So, a candidate can actually lose a numerically larger number of state primaries but still win the nomination if he or she won enough states with large numbers of delegates. (Now, each party's delegate system is slightly different, and each state awards delegates slightly differently, but my general comments are accurate.)
3) Residents of Washington, DC hold a primary but Puerto Rico does not. Residents of both places may vote in the general election, however.
4) Normally a sitting president does not have a challenger for his (all men so far) party's nomination, so the president does not compete in the primaries. But whether a president has a challenger for the nomination is determined by circumstances more so than by tradition and not at all by party policy. There is nothing stopping someone from challenging a sitting president. Significant recent examples include Pat Buchanan who competed against the first President Bush in 1992; Ronald Reagan, who challenged President Ford in 1976; and - most notably perhaps - Eugene McCarthy, who challenged President Johnson in 1968, in opposition to the war in Vietnam.
You're certainly right about the fundamental differences between the UK and the US. You may be interested to know that in his doctoral dissertation at Johns Hopkins University, future US president Woodrow Wilson compared the parliamentary system of the UK with the Congressional system of the US, and concluded that the parliamentary system was better. Less than 30 years later as president, given his friction with Congress about the League of Nations, he probably felt the same but more so!
I hope that this is helpful, Mark!