Existential vs existing

The meanings of the two words are different, they cannot be switched without changing the meaning of the statement.

• Existential relates to existence itself. An existential threat is something that threatens the existence of something. As S Conroy points out:

  If he is an existential threat to the democratic [sic] establishment then he threatens [the establishment's] existence.

  Note: I've retained the capitalization of the quoted comment, but 'Democratic' should be capitalized based on how it is used in the question. "Democratic" refers to the political party. Whereas "democratic" refers to the governance system.

• Existing describes something that exists. An existing threat is a threat that exists, as opposed to one that doesn't, like an imaginary one. As S Conroy explains:

  If he is an existing threat, then he is a threat that exists now; we don't know how serious the threat is or whether or not it is existential.