With President Trump a focus of the race, Ms. James overcame a challenge from a Republican former federal prosecutor, Saritha Komatireddy.

Sand is the first Democrat to win a gubernatorial election in more than two decades.

Democrats last gained a majority of governorships in 2006, flipping six governors’ offices in the last midterm under a second-term Republican president. Republicans recaptured the majority four years later. Meanwhile, more states under Democratic leadership could mean more governors fighting — or ignoring — Trump’s agenda.

David Crowley won the Democratic nomination after dropping out of the primary and then jumping back in.

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