After weeks of intense media speculation and sustained pressure from Democratic lawmakers, major donors, and senior advisors, President Joe Biden has announced that he is bowing out of the presidential race. He is the first sitting president to step aside so close to Election Day. “I believe it is in the best interest of my party and the country for me to stand down and focus entirely on fulfilling my duties as president for the remainder of my term,” Biden said in a letter on Sunday.

He endorsed his vice president, Kamala Harris, to take his place. “Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year,” he said in another statement. Not long after, Harris announced via the Biden campaign that she intends to run for president. “I am honored to have the president’s endorsement and my intention is to earn and win this nomination,” she said.

During his term, President Biden managed to shepherd a surprising number of major policies into law with a razor-thin Democratic majority in the Senate. His crowning achievement is signing the Inflation Reduction Act, or IRA — the biggest climate spending law in U.S. history, with the potential to help reduce greenhouse gas emissions up to 42 percent below 2005 levels by 2030. In announcing his withdrawal, Biden called it “the most significant climate legislation in the history of the world.”

Despite his legislative successes, the 81-year-old Democrat couldn’t weather widespread blowback following a debate performance in June in which he appeared frail and struck many in his party as ill-equipped to lead the country for another four years. He will leave office with a portion of his proposed climate agenda unpassed and the U.S. still projected to miss his administration’s goal of reducing emissions at least 50 percent by 2030.

Former president Donald Trump has vowed to undo many of the policies Biden accomplished if he becomes president, including parts of the IRA. And scores of his key advisors and former members of his presidential administration contributed to a blueprint that advocates for scrapping the vast majority of the nation’s climate and environmental protections. Whichever Democrat runs against Trump has a weighty mandate: protect America’s already-tenuous climate and environmental legacy from Republican attacks.

With Biden’s endorsement, Vice President Harris, a former U.S. senator from California, is the favored Democratic nominee, but that doesn’t mean she will automatically get the nomination. There are fewer than 30 days until the Democratic National Convention on August 19. The thousands of Democratic delegates who already cast their votes for Biden will either decide on a nominee before the convention, or hold an open convention to find their new candidate — something that hasn’t been done since 1968.

As vice president, Harris argued for the allocation of $20 billion for the EPA’s Greenhouse Gas Reduction Fund, aimed at aiding disadvantaged communities facing climate impacts, and frequently promoted the IRA at events, touting the bill’s investments in clean energy jobs, including installation of energy-efficient lighting, and replacing gas furnaces with electric heat pumps. She was also the highest-ranking U.S. official to attend the international climate talks at COP28 in Dubai last year, where she announced a U.S. commitment to double energy efficiency and triple renewable energy capacity by 2030. At that same conference, Harris announced a $3 billion commitment to the Green Climate Fund to help developing nations adapt to climate challenges, although Politico reported that the sum was “subject to the availability of funds,” according to the Treasury Department.

“Vice President Harris has been integral to the Biden administration’s most important climate accomplishments and has a long track record as an impactful climate champion,” Evergreen Action, the climate-oriented political group, said in a statement.

Harris caught some flak for using a potentially overstated “$1 trillion over 10 years” figure to describe the Biden administration’s climate investments. She got that sum from adding up all of the administration’s major investments over the past four years, some of which are only vaguely connected to climate change.

As a presidential candidate in 2019, Harris proposed a $10 trillion climate plan to achieve carbon neutrality by 2045 on the campaign trail, including 100-percent carbon-neutral electricity by 2030. Under the plan, 50 percent of new vehicles sold would be zero-emission by 2030; and 100 percent of cars by 2035. But that proposal, like similarly ambitious climate change proposals released by other Democrats during that election cycle, was nothing more than a campaign wishlist. A better indicator of what her plans for climate change as president would look like — better, even, than her record as vice president, as much of her agenda was set by the Biden administration — could be buried in her record as San Francisco’s district attorney from 2004 to 2011 and as California attorney general from 2011 to 2017.

As district attorney, Harris created an environmental justice unit to address environmental crimes affecting San Francisco’s poorest residents and prosecuted several companies including U-Haul for violation of hazardous waste laws. Harris later touted her environmental justice unit as the first such unit in the country. An investigation found the unit only filed a handful of lawsuits, though, and none of them were against the city’s major industrial polluters.

As attorney general, Harris secured an $86 million settlement from Volkswagen for rigging its vehicles with emissions-cheating software and investigated ExxonMobil over its climate change disclosures. She also filed a civil lawsuit against Phillips 66 and ConocoPhillips for environmental violations at gas stations, which eventually resulted in a $11.5 million settlement. And she conducted a criminal investigation of an oil company over a 2015 spill in Santa Barbara. The company was found guilty and convicted on nine criminal charges.

“We must do more,” Harris said late last year at the climate summit in Dubai. “Our action collectively, or worse, our inaction will impact billions of people for decades to come.”

Clayton Aldern contributed writing and reporting to this article.