Usage patterns
〈人〉が党首に選ばれるto be elected (as) party leader
He was elected the new party leader last month.
党首に就任するto take up the post of party leader
She took office as party leader this year.
党首を辞任するto resign as party leader
He resigned as party leader after losing the election.
党首として〈動詞〉to do something in one's capacity as party leader
He held a press conference in his capacity as party leader.