With no incumbent in the race, next year’s contest to replace Philip D. Murphy as governor of New Jersey was never going to be dull.

But wild-card factors that might have seemed far-fetched a year ago have combined to unleash something rare in the state: a competitive race with an unpredictable outcome.

The list of well-known contenders for the seat already includes two members of Congress, the mayors of New Jersey’s two largest cities, the longest-serving State Senate president and the head of a powerful teachers’ union. Two other candidates cut their teeth in the State Legislature. There is also a popular far-right radio host and a former state senator running as “Ed the Trucker.”

President-elect Donald J. Trump’s surprisingly strong showing in November in reliably Democratic New Jersey and a sweeping change to the rules for conducting primary elections have undone many of the state’s old political assumptions, leaving even seasoned observers riveted.

“It sure is different from the script we’re used to,” said Peter J. Woolley, director of the School of Public and Global Affairs at Fairleigh Dickinson University.

The field of candidates is unlikely to thin significantly before June’s Democratic and Republican primaries, where the winning margins are expected to be narrow.