Harris is strong on environmental justice, climate activists say.

Climate activists praised the selection of Ms. Harris, saying the move signaled a sustained Democratic focus on environmental justice.

Ms. Harris was an original co-sponsor of the Green New Deal, an expansive plan to curb planet-warming greenhouse gas emissions while also addressing economic inequality. She supports abolishing the Senate filibuster if Republicans stand in the way of passing climate change legislation. And she has called for a $10 trillion increase in spending over a decade as well as a price on carbon dioxide pollution, with a dividend that is returned directly to households.

Ms. Harris’s core environmental focus has consistently been on how poor communities are disproportionately affected by polluting industries.

A former prosecutor, Ms. Harris vowed to maximize the power of the legal system to punish corporate polluters. Last year she laid out specific plans for protecting vulnerable communities, including establishing an independent Office of Climate and Environmental Justice Accountability and scoring environmental regulations or legislation based on how they affect low-income communities.

“I’m super hopeful,” said Catherine Flowers, a senior fellow at the Center for Earth Ethics. The choice of Ms. Harris, she said, “is elevating the conversation and making it a priority.”