Former vice president Joe Biden speaks to voters during a campaign stop at the Community Oven in Hampton, N.H., on May 13, 2019. (Melina Mara/The Washington Post)

Joe Biden has spent the past two days attempting to push back against liberal criticism over his stance on climate change, pointing to decades-long environmental work in an effort to blunt the first major ideological skirmish of his presidential campaign.

On its face, the debate centers on his climate change agenda, one that activists are preemptively criticizing before Biden formally releases his plan this month. But on a deeper level, the criticism drives at Biden’s political philosophy to seek the types of consensus-building compromises that many in the Democratic base say they are in no mood to make.

Biden’s approach represents a fundamental test of whether the Democratic Party has shifted out of his grasp in recent years — to one that punches back hard against the Trump-controlled Republican Party with a sharply liberal agenda — or whether it is one that wants a return to bipartisan bonhomie.

Biden often preaches the merits of working with Republicans — calling them good and decent at their core — and calls President Trump an aberration.

“The thing that will fundamentally change with Donald Trump out of the White House, not a joke,” he said on Tuesday in New Hampshire, “is you will see an epiphany occur among many of my Republican friends.”

“If we can’t change it, we’re in trouble,” he added. “This nation cannot function without generating consensus. It can’t do it.”

He told a story about learning to respect Republicans he served with during his 36 years in the Senate. And he launched into extended remarks lamenting the time he discovered that a private dining room for senators had been converted into a room with just lounge chairs.

“Folks, you gotta get to know the other team,” he said. “Get to know them personally.”

Biden has yet to announce many policies — repeatedly telling crowds that he would get into more detail except he doesn’t want to force them to stand for too long. But in some areas he seems prepared to move to the left of the Obama administration, if keeping to the right of some of his primary rivals.

On health care, for example, he has been open to a public option that would give Americans a choice to buy into a Medicare-like health insurance plan. That option goes beyond the Affordable Care Act but falls short of the Medicare-for-all plan pushed by others.

But climate change has emerged as one of his key early tests.

The tension was on display Monday night as a large crowd of liberal activists repeatedly booed references to a Reuters report that the former vice president would seek a “middle ground” on what they consider a defining issue among the crowded Democratic field.

At a rally for the Green New Deal attended by about 1,500 activists at Howard University, speaker after speaker criticized Biden’s reported position, which Reuters attributed to a campaign adviser.

“What we learned from the Obama administration is unless we find middle ground on these issues, we risk not having any policies,” Heather Zichal, one of Biden’s advisers on climate change, told Reuters. Zichal did not respond to several requests for comment from The Washington Post.

Biden has since pushed back against the report, and has promised a major speech later this month outlining his environmental priorities. But that did little to quell concerns among those who attended the event organized by the left-wing Sunrise Movement, including Sen. Bernie Sanders (I-Vt.), who is also seeking the Democratic presidential nomination, and Rep. Alexandria Ocasio-Cortez (D-N.Y.).

“Who here liked it when Joe Biden said he was middle of the road on climate policy?” asked Alexandra Rojas, executive director of Justice Democrats, a left-wing campaign group that is attacking Biden on social media.

Ocasio-Cortez, who is neutral in the presidential race but has repeatedly teamed up with Sanders, also went after Biden without using his name. After she criticized “conservatives in both parties” who did not have a comprehensive plan to transition from fossil fuels, an audience member shouted, “No middle ground!”

“No middle ground is right!” Ocasio-Cortez said. “I will be damned if the same politicians who refused to act [earlier] come back today and say we need a middle-of-the-road approach to save our lives.”

Biden, who offered an initial defense Monday evening before Ocasio-Cortez spoke, responded more fully on Tuesday morning.

“Look at my record. She’ll find that nobody has been more consistent about taking on the environment and a green revolution than I have,” he said. “And so, look, anyways — I don’t think she was talking about me.”

Biden has often pointed to remarks he gave in 1987, a time when climate change was not on the national radar and before Ocasio-Cortez was born.

“Global warming, should it occur in accord with the direst predictions, would be a catastrophe of biblical proportions for the entire world,” he said in a Senate floor speech. “The human activities that could bring it about — the inefficient burning of fossil fuels . . . the destruction of tropical forests — are occurring right now. And unless these activities are changed in the next few years . . . a disastrous and irreversible warming could become inevitable.”

He pushed legislation that would form a task force that would be mandated to study global warming and come up with recommendations for further action.

“It’s a long campaign. And everybody should just calm — they should just calm down a little bit,” Biden said Tuesday as he munched on a muffin in Concord, N.H. “Take a look at my record from before. There’s been nothing middle of the road about my record dealing with the environment. Nothing.”

In a statement to The Post, former secretary of state John F. Kerry, the U.S. negotiator for the Paris climate accords — the multinational agreement to limit greenhouse emissions — praised Biden.

“Joe is ambitious on climate and energy,” Kerry said. “The ‘middle’ they’re talking about is building a new climate coalition that unites true believers with blue collar workers who will build the energy of tomorrow.”

“Joe has been a leader on climate change since the first climate bill in 1987,” he added. “Everyone should wait to see his plan. His record has earned him that much and more.”

It’s unclear how close Biden’s new plan will come to the goals embodied in the Green New Deal, a plan introduced by Ocasio-Cortez and Sen. Edward J. Markey (D-Mass.) that envisions the United States achieving net-zero greenhouse gas emissions within a decade while also guaranteeing Americans high-paying jobs and high-quality health care.

The plan has won broad support among Democratic activists — while earning less support among Democrats overall — and has been endorsed by several of the party’s presidential hopefuls. But it has been heavily criticized by Republicans, who argue that it is unrealistic and part of a lurch toward socialism by Democrats.

In New Hampshire, Biden, as he has on several issues during his first few weeks as a candidate, pointed to the accomplishments during the Obama administration in which he served. He cited investments made in clean energy as part of economic stimulus legislation, as well as an increase in the fuel efficiency standards for passenger vehicles.

“Why in God’s name did this president freeze that?” Biden said, referring to Trump. “Even the automobile companies were for it. Because they know they can’t compete in the world without having more energy efficient cars and electric vehicles.”

Sanders, who like Ocasio-Cortez spoke at Monday night’s rally in Washington, did not criticize Biden by name. But he criticized candidates who he said had only a tepid commitment to fighting climate change.

“We have an overall economy that is rigged and a political system that is corrupt,” he said. “A lot of people, say, ‘Well, I want to combat climate change, but, you know, I don’t want to take on the fossil fuel industry.’ That is not the way it happens.”

Sanders took more direct aim at Biden last week after the Reuters report was published.

“There is no ‘middle ground’ when it comes to climate policy,” he said in a tweet. “If we don’t commit to fully transforming our energy system away from fossil fuels, we will doom future generations.”

Other 2020 hopefuls also referred to the report as they sought to distinguish themselves from Biden.

“ ‘Middle-ground’ approaches and half measures won’t cut it,” Washington Gov. Jay Inslee (D), who has put climate change at the center of his presidential bid, said in a tweet. “We need a large-scale national mobilization to defeat climate change and grow millions of jobs in a clean energy economy.”

Some in the crowd Monday night suggested a plan from Biden that falls short of the Green New Deal would be disappointing, but not necessarily a dealbreaker.

“If Biden is the nominee and he never supported or never signed a pledge to support the Green New Deal, it would it be tough to vote for him, but I’d do it,” said Elly Ren, 18, a student at Johns Hopkins, as she cut out images of Sanders and Ocasio-Cortez for a poster. “I guess we’d just have to put pressure on him in the White House.”