Did you mean, “how many states don’t care which party you belong to if you vote in a primary”? Like Santorum encouraging Democrats in Michigan to come vote for him (to say he could beat Romney in his home state)? Or the Democrats who were encouraging other Dems to vote Santorum in the primary because they thought he would be the easiest target for Obama to beat?
Some states allow “cross-over voting” in primaries. Some don’t. Some allow “independent” (no registered party) to pick a party to vote in the primary, some don’t. Your Mileage May Vary, depending on state.
You need to be registered as a voter to vote in the actual elections. (From what I’ve read, up here in Canada). This is a good and a bad thing, because if you don’t get around to registering in time (what, weeks before the election? Months?) you can’t vote. Voter registration does NOT require you to identify a political party.
Much of the debate about voter registration and needing ID to vote centers around the Democrat contention that the Republicans are trying to block votes by poor people, felons, or minorities, who supposedly tend to vote Dem, and Republican fears that the Dems will trot out invalid, ineligible voters by the busload to stuff the ballot boxes.
Indeed, many locations are famous (both parties) in ensuring in the past, that for example everyone in the cemetery who was eligible to vote, did so, early and often…
I saw an interview with Bill Clinton a few years ago (just after the last election, I think). In presidential elecion, he said, there used to be a core set of voters, about 40% each side, who would vote for their party anyway. The middle 20% were independent/undecided and those were the ones who had to be persuaded every 4 years to decide the election. Lately, he said, the ratio had altered slightly and there was about 45% Rep, 40% Dem and 15% undecided. Of course, not all those voters are registered paid up party members.
As mentioned above, the winner-take-all per state process and the electoral college meant that only a few states (Ohio, Florida, and Pennsylvania are mentioned often) are usually close enough to need serious campaigning. The others, like CA, NY, MA, TX, etc. are usually to much of an uphill battle for the other candidate.
The “McCain-Romney-Giuliani” problem is that the more fantical voters tend to care and participate in the primary process; so a more extreme viewpoint is likely to get these votes, especially in the Republican process. Those candidates, for example, were moderates with a firm belief in issues like good health care laws and in favour of abortion until they realized that would effectively exclude them from winning the Republican nomination. However, it’s a balancing act. The more they pander to conservatives, the less they will appeal (and the more vulnerale to attack ads) when trying to persuade that last 15% of undecided. however, if they are too middle-of-the-road, they will turn off the more right-wing “true believers” in their party, who may not come out to vote. The Dems have similar problems with abortion rights and women’s/minority rights, union rights, and other issues dear to the heart of their more fanatical members.
That points out the BIGGEST flaw in the US system - about 55% of the voters actually vote. The purpose of those big candidate campaign offices everywhere, the voter registration drives, the phone polls and door to door, is to identify those who say they will vote for your candidate, then try their best to get those people out to the polling station on election day.