Can an incumbent president be primaried?

Related questions

1. How does the caucus process work?
2. Has any president not run for a second term?
3. Are primaries or caucuses better?
4. Is Vice President Biden running for president?
5. Will Republicans primary Trump in 2020?
6. What does Iowa caucus mean?
7. How many presidents have been impeached?
8. Does an incumbent president have to run in the primaries?
9. What is the 2020 primary schedule?
10. Has an incumbent president ever lost a primary?

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