Nominalize

v. t.
  1. To convert into a noun; as, a nominalized sentence; a nominalized adjective or verb.

In a sentence of the form: “That Kennedy would have been a great president if he had not been assassinated is one of the enduring myths of the Camelot conceit.“ the sentence “Kennedy would have been a great president if he had not been assassinated” has been nominalized by the introductory “that”.