Governor Hochul fortified her frontrunner status in the governor’s race as she formally received the endorsement of the state’s Democratic party. 

Senator Clinton introduced Mrs. Hochul, who hails from the city of Buffalo in western New York, at a convention in Manhattan. 

“I’ve known Kathy for a long time and I can tell you something everybody is learning: No one will work harder for the people of the Empire State. Every county, every community. She is a governor for all of us,” Mrs. Clinton said Thursday. 

The first female presidential nominee of a major political party, in 2016, Mrs. Clinton said it was time for another first in New York — to elect a woman governor for the first time.

Before Mrs. Hochul spoke, the women embraced on stage and Mrs. Clinton lifted Mrs. Hochul’s hand aloft to the applause of several hundred Democrats.

Mrs. Hochul called for party unity, saying it was the “greatest threat to the Republican Party,” and “their biggest nightmare.” 

She also vowed, as the de-facto leader of the state Democratic Party, that Democrats would follow her “playbook for success.”

“Here’s my playbook: You run with confidence, but with the tenacity of an underdog. You take nothing for granted and you fight until the very last second,” she said. 

As New York’s lieutenant governor, Mrs. Hochul was relatively unknown when she became the state’s first female governor in August, taking over when Governor Cuomo resigned amid allegations he sexually harassed several women. 

Mrs. Hochul was initially expected to face a crowded field of challengers, including a tough contest against Attorney General Letitia James. But the new governor corralled key Democratic endorsements and stockpiled nearly $22 million for her campaign. Ms. James dropped out of the race to run for re-election, and Mrs. Hochul became the party’s leading contender. 

In an interview Thursday afternoon, Mrs. Hochul said she understands she caught some people by surprise but said she’s been underestimated before.

“I don’t say that with arrogance. I just say it with the knowledge that no one will ever outwork me. No one knows what I know about this state and the relationships that I’ve cultivated,” she said.

Those relationships date back to Mrs. Hochul’s time as a county clerk more than a decade ago and serving in the U.S. House of Representatives, she said. 

“It was just good old-fashioned working hard and doing what we had to do to raise our name recognition. And now we’ll have the resources to go the distance,” she said. 

Despite winning the state party’s official support, she’s still expected to face challenges in the June primary from New York City’s elected public advocate, Jumaane Williams, the most progressive candidate in the race, who has won backing from the liberal Working Families Party. She also faces a challenge from Representative Tom Suozzi, a moderate who represents Long Island and whose politics hew closely to Mrs. Hochul’s. 

Mr. Suozzi told reporters Thursday morning that Mrs. Clinton had recently phoned him and encouraged him to drop out of the race, but he declined. 

Mrs. Hochul said in the interview that she did not ask Mrs. Clinton to make the call to Mr. Suozzi and did not know about it in advance.