WELLINGTON, New Zealand — A minor party holding the balance of power in New Zealand threw its support behind 37-year-old Jacinda Ardern’s bid to become prime minister on Thursday, ending the center-right National Party’s nine-year grip on power.

Ms. Ardern, the leader of the Labour Party, will take over from Prime Minister Bill English, reversing Labour’s long-flagging fortunes and giving the country its youngest leader in more than 150 years.

Ms. Ardern’s ascension represents a remarkable rise for a woman who just months ago became Labour’s youngest leader ever, setting off “Jacindamania” among the party’s followers. Unconventional and described by colleagues as intensely focused, she has defied the norms of New Zealand politics and refused to be pinned down on whether she has considered having children, saying no male politician would be forced to answer that question.