Ballot access refers to the rules and regulations determining which candidates and parties may appear on voters' ballots. Rules for ballot access are complex and state-specific. Generally, a candidate may run as the nominee of a political party, as an independent candidate, or as a write-in candidate.
"Sore loser" laws
A voting machine at the Smithsonian National Museum of American History
Some states bar candidates who sought, but failed, to secure the nomination of a political party from running as independents in the general election. These rules are often refered to as "sore loser laws." According to ballot access expert Richard Winger, 45 states have sore loser laws on the books, but in 43 of these states the laws do not seem to apply to presidential candidates.