Hawkeye Primaries

21 states, including Mississippi, have classic open primaries, in which each voter picks a party on primary day.

Iowa is the only one of the 21 states that registers voters by party.

Even if you are not registered with one of the political parties, you can register at the polling place on the day of the primary and vote in that particular party’s election. It’s that easy.

The article fails to mention that a registered Democrat who wants to vote in the Republican primary may do so by changing his registration at the polling place on primary day. The same, to be sure, is true of a registered Republican who wants to vote in the Democratic primary (a voter, of course, can only participate in one party's primary).

It's my understanding that the system is the same for Iowa's quadrennial, first-in-the-nation presidential caucuses. A voter who is registered with the opposing party simply goes to the site of the party's caucus in which he wishes to participate, where he changes his registration.

Other states have same-day registration for voters not already registered, but Iowa is the only state that permits a voter to switch parties at the polling place (or caucus site) and vote in the primary or caucus of his new party.