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It’s been nearly 70 years since there last was a presidential election rematch, but it has occurred more than people might think in U.S. history.

Dissatisfied Democratic primary voters in Michigan have cast enough protest votes to deny President Joe Biden two of the state’s 117 delegates.

As the Republican presidential primary intensifies this summer, most White House hopefuls are devoting their time to events in Iowa and New Hampshire, the states that will kick off the nomination process early next year.

Even if one candidate has more delegates than the other, that’s only a “plurality,” not a majority, per party rules. Candidates who earned delegates, but have since dropped out of the race, can try to push their delegates onto another candidate. A candidate needs to earn 2,375 delegates on subsequent ballots to win the nomination. However, having a contested convention is something the Democratic National Committee will do its best to avoid in the coming months. The last time a contested convention occurred was in 1952.