When Gov. Kathy Hochul of New York chose Antonio Delgado to be her lieutenant governor in 2022, she had nothing but the highest praise for her new No. 2.

She said Mr. Delgado, then a rising Black political star who represented a competitive House district, was a “battle-tested campaigner.” She praised his work ethic and said that his ability to “unite communities” would serve her administration’s goals.

Three years later, their partnership has completely disintegrated. After months of open political warfare with Ms. Hochul, Mr. Delgado said Monday he would challenge her in next year’s Democratic primary.

“People are hurting and New York deserves better leadership,” Mr. Delgado said in an interview. “There’s an absence of bold, decisive, transformational leadership.”

That phrase has become the theme for Mr. Delgado’s campaign, the first official challenge to Ms. Hochul, and a message he repeats in a video announcing his candidacy.

He never mentions Ms. Hochul by name in the video, but he suggests that everyday New Yorkers deserve a governor with a vision to fight for them as they face unaffordable housing, child care and health care costs.