Can an incumbent president be challenged in a primary?

Related questions

1. What is the 2020 primary schedule?
2. Are primaries or caucuses better?
3. Does an incumbent president have to run in the primaries?
4. What is the point of a caucus?
5. Can an incumbent president be primaried?
6. Has any president not run for a second term?
7. Is Vice President Biden running for president?
8. When was Donald Trump sworn in?
9. Has an incumbent president ever lost a primary?
10. What does it mean to caucus for a candidate?

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