President Obama and John Podesta in May 2014. (Pablo Martinez Monsivais/AP)

John D. Podesta was not one of the founding members of President Obama’s tight inner circle, and after about a year on the White House’s payroll, he has left the administration. But apart from the president himself, no single person may be more responsible for shaping the ambitions of Obama’s second term.

Mostly, he did this by showing the president how to fight back against GOP antagonists on Capitol Hill. After a contentious first term, in which the White House repeatedly became stymied and frustrated by congressional Republicans and its own mistakes, Podesta helped orchestrate aggressive countermeasures that involved the expanded use of executive authority to get around Congress and the implementation of a starkly more liberal agenda.

As the White House gears up for the final rounds of fights over health care, climate change and immigration with the GOP-led Congress and in the courts, it is following a playbook that is largely Podesta’s.

“He was an incredibly important voice in this White House on all matters,” said senior White House adviser Dan Pfeiffer, who is leaving next month. “He’s left, but the imprint he’s left will last long beyond his departure.”

Podesta’s approach has energized and focused Obama and heartened Democrats nationally, but it also has further alienated and antagonized Republicans, who now control Congress. And after Democrats suffered a rout in last year’s midterm elections, it’s unclear how the strategy will play in the next election — a battle Podesta is about to enter as a top adviser to likely candidate Hillary Rodham Clinton.

White House National Security Adviser Susan Rice, right, and counselor to the president John Podesta are in the audience at a Feb. 9 news conference held by German Chancellor Angela Merkel and U.S. President Barack Obama at the White House. (Alex Wong/Getty Images)

Podesta has been an informal adviser to Obama for years and ran his transition team, but it was during one of the administration’s lowest points — after the shaky rollout of HealthCare.gov in 2013 — that Obama persuaded Podesta, now 66, to help revamp the White House’s operations from the inside.

In an administration once notable for its youthfulness and relative inexperience, Podesta was immediately the respected, elder wise man.

Pfeiffer, who pushed for an executive-oriented strategy in an internal White House memo, described him as a kind of Obi-Wan Kenobi figure. “Even after he’s gone, his apparition will return to provide wise, sage advice,” he joked.

The president recruited Podesta to oversee the implementation of his climate action plan, including negotiations with other major greenhouse gas emitters such as China and India, as well as rules to limit such emissions from new and existing power plants, heavy-duty vehicles, and oil and gas production. The Environmental Protection Agency will finalize rules for power plants this year and is poised to propose regulations on the two other fronts within months.

“His legacy will be the creation of a regulatory and political strategy to address greenhouse gas emissions that, following the collapse of legislation, opened up multiple fronts of attack on the fossil-fuel industry,” said Stephen Brown, vice president of government relations at the oil refiner Tesoro. “That said, he will also share some degree of responsibility for the raw partisanship that now characterizes any discussion of this and other energy or environmental policies.”

Podesta’s influence stems partly from the infrastructure he has built for the Democratic Party. The liberal think tank he founded, the Center for American Progress, has employed White House officials including Chief of Staff Denis McDonough, senior adviser Brian Deese and Christy Goldfuss, acting director of the Council on Environmental Quality.

But Podesta also spent years working on Capitol Hill and later founded a lobbying firm with his brother, Tony, before serving as President Bill Clinton’s chief of staff for his last two years in office.

That range of experience gave him a deep working knowledge of Washington and has been put to use in Obama’s second term — one hallmark of which has been to use the levers of government for political and policy advantage and to court confrontation with adversaries.

The result has been presidential executive action on immigration, paid leave, the minimum wage and other Democratic priorities, at a time when Obama’s legislative proposals have stood no chance of becoming law.

On his last day at the White House, Podesta said he was confident that Obama would veto any attempt to roll back the climate-change policies he has put in place over the years.

“There’s no must-pass bill,” he said, sitting in an office already stripped of his pictures of barracuda and mahi-mahi. “The Republicans have shut the government down on multiple occasions in the past, so they’re capable of that, but they’d be wise to not test him on it.”

Robert Dillon, a spokesman for the Senate Energy and Natural Resources Committee, said lawmakers have seen “unrelenting regulatory aggression” from the administration since Podesta came aboard. “This could be that the president is showing his true colors as his second term winds down or that he’s been steered in this direction by Mr. Podesta.”

Podesta said the administration tried unsuccessfully to forge a bipartisan agreement on climate legislation in the Senate in 2010. And the fact that the economy has rebounded as renewable energy has grown in the United States, he argued, proves that the two can coexist.

“But now, I think, when you put someone in charge of the Environment Committee who thinks climate change is a hoax, our prospects for finding bipartisan solutions tend to get reduced,” he said, laughing, referring to Sen. James M. Inhofe (R-Okla.), the committee chairman. “So we’re just going to have to do what we need to do, using existing authorities, and I think if there’s opportunity to find common-sense solutions, we’re certainly open to it. But if we have to sit with our hands folded until the climate deniers on Capitol Hill change their mind, we’re not going to do that.”

Podesta’s departure is not seen as an end so much as a continuation, given his role in Clinton’s nascent 2016 campaign. Republicans argue that a Clinton presidency would amount to an extension of the Obama administration.

“Bringing on John Podesta shows Hillary Clinton will continue President Obama’s obstruction of the Keystone Pipeline and the EPA’s regulatory onslaught that helped deliver the slowest recovery in modern history,” Michael Short, a spokesman for the Republican National Committee, said in an e-mail. “And more importantly, this Washington-knows-best approach hurts the poor and middle class the most, driving up their bills while simultaneously diminishing their job prospects. Voters overwhelmingly don’t want a third term for President Obama’s liberal agenda, but it’s clear that’s what Hillary Clinton and John Podesta intend to give them.”

Although Podesta worked on a range of issues, including big data and precision medicine, he made his biggest mark in lands protection. During Obama’s first term, the Center for American Progress published a study showing that the administration had leased far more land for energy extraction than it had protected. At the time, Podesta told The Washington Post in an interview that there was “little bandwidth in the White House to think about” land conservation.

In the past year the president has declared seven national monuments, nearly as many as he did in the preceding five years. Altogether, he has protected more than 260 million acres of public lands and waters — more than any other president in history.

Podesta noted that he had little exposure to the outdoors as a kid, saying: “I grew up on a street corner in Chicago. I don’t know what was flowing through the sewers underneath that street corner; there probably wasn’t a lot of wildlife that ever became iconic fauna and flora.”

But he spent time during and after college working on campaigns in Colorado, which won him over to the idea of protecting the environment. Each time he served in the West Wing, he lobbied the occupant of the Oval Office to make large swaths of public lands off-limits to development.

“I had that experience with President Clinton,” he recalled. “Once you get out to visit some of the places that are really spectacular in this country, and understand that you can have a hand in not only remembering how they connected to the history of our country but to protect them for future generations, presidents kind of get into that. So I think President Obama’s no exception.”