Does an incumbent president have to run in the primaries?

Related questions

1. Has Joe Biden declared for 2020?
2. Has an incumbent president ever lost a primary?
3. What is the name of the Vice President of the United States now?
4. When did Joe Biden run for president?
5. Will Republicans primary Trump in 2020?
6. Has any president not run for reelection?
7. What does Iowa caucus mean?
8. When was Donald Trump sworn in?
9. Can a two term president run for vice president?
10. Is Vice President Biden running for president?

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25. #opposition
26. #states