The notification letter to the U.N. set off a yearlong process that would officially pull the U.S. out a day after the 2020 presidential election. Even if a climate-friendly Democratic candidate wins the White House, re-entry wouldn’t necessarily be a smooth process.

Rationale: Mr. Trump, who has mocked climate science as a hoax, holds that the accord would cripple growth and intrude on U.S. sovereignty.

Resistance: Environmentalists are pressing U.S. states, cities and businesses to cut emissions and move to renewable energy, and hundreds of local governments and businesses have made pledges under a movement called We Are Still In. And some Democratic presidential candidates had harsh words for the withdrawal, a position that could appeal strongly to younger voters.

Global strategy: Nearly 200 countries signed the accord to slash greenhouse gas emissions and help poor countries cope with climate disruptions. Making the arrangement work without the U.S. would require new efforts by major polluters like China and India, whose capital is currently swamped with poisonous air.