Dates and deadlines
Note that Iowa does not have a presidential primary and presidential candidates will not be on the June 2 ballot.
A primary is an election that takes place within each official political party. No one is elected in a primary election. Instead, voters affiliated with political parties will select their party's candidates for the November
3 general election.
Only the full-status parties hold primaries, and only voters who register with a party may vote in the primary. Voters may change party at any time including Election Day. Two parties will have primaries in 2018: the Democratic and Republican parties.
The Libertarian Party, which had a primary in 2018, has lost full party status and will not have a primary. The Libertarians and Greens have "organization" status. People can register with those parties, but they do not have primaries.
Voters can only participate in one party's primary.
If you are registered to vote as a Democrat, for example, your vote in the Primary Election will help choose the Democratic Party's nominees for various offices. Since you, as a registered Democrat, are selecting your party's candidate to run against candidates from other parties in the General Election, you will be sent a ballot with only Democratic candidates to choose from. You will not see the names of any Republican candidates on your Primary Election ballot.
In the General Election, which decides who will actually fill the offices, all voters get ballots listing all of the candidates.
Can an independent vote in a primary?
Yes and no. You can walk IN as an independent, but you can't walk OUT as an independent (the official term is "no party"). You have to declare affiliation as a Democrat or Republican when you request your ballot. Overseas voters who register as No Party, Green or Libertarian will not be sent a ballot for the June 2 primary.
Can I vote for a Republican for one office and a Democrat for another?
In a General Election, yes. In a Primary Election, no.
Can I write in a candidate of one party in the other party's primary?
Yes, but the party primaries are separate contests. If you vote in the Democratic primary and write in Abe Lincoln, that will be counted as a Democratic primary vote for Lincoln, but it will not be added to Lincoln's Republican primary total. Candidates can only accept the nomination of one party.