What's the difference between a primary and a general election?

The three main systems

States use meaningfully different primary rules:

• Closed primary — only voters registered with the party may vote in it. Unaffiliated voters are excluded.
• Open primary — any voter may participate in one party's primary, choosing which on Election Day.
• Semi-closed — party members plus unaffiliated voters may participate; members of other parties may not.
• Top-two or top-four — all candidates appear on one ballot regardless of party, and the leading finishers advance to the general election, sometimes both from the same party.

Why the primary is often the real election

In districts that lean heavily toward one party, the general election is a formality and the nominee effectively wins the seat in the primary. Primary turnout is typically far lower than general election turnout, so those seats are decided by a small fraction of the electorate.

This is especially pronounced in state legislative races and in local contests, where a single-party district may see no opposing candidate file at all.

Runoffs

Some states require a candidate to reach a threshold — often a majority — to win a primary outright. If no one does, the top finishers meet in a runoff several weeks later.

Runoff turnout is usually dramatically lower than in the initial primary, which makes those the lowest-participation, highest-leverage elections most voters never hear about.