Here's why:
* Linking verbs connect the subject of a sentence to a noun, pronoun, or adjective that renames or describes the subject.
* In this case, "he" is the subject, and "senator" is the noun that renames the subject.
* "Became" acts as the bridge, linking "he" to his new role as "senator."
The second part of the sentence ("but then decided to resign") uses the action verb "decided."