President Trump traveled on Wednesday to the new political battleground of Georgia to blast away at one of the nation’s cornerstone conservation laws, vowing to speed construction projects by limiting legally mandated environmental reviews of highways, pipelines and power plants.

One day earlier, his Democratic presidential rival, Joseph R. Biden Jr., took a different tack, releasing a $2 trillion plan to confront climate change and overhaul the nation’s infrastructure, claiming he will create millions of jobs by building a clean energy economy.

In that period, the major party candidates for the White House displayed in sharp relief just how far apart they are ideologically on infrastructure and environmental matters of vital importance to many American voters, particularly in critical battleground states, including Pennsylvania and Florida.

Mr. Biden is trying to win over young voters and supporters of his vanquished rival, Senator Bernie Sanders of Vermont, by showing an aggressive awareness of climate change and promising to move urgently to combat it. At the same time he has sought to maintain his promised connection to white, working class voters, especially in the Upper Midwest, who swung to Mr. Trump four years ago and are leery of what they see as threats to their livelihood, especially jobs in the oil and gas industry.

The president, in contrast, is pretty much where he has been for more than a decade: intermittently acknowledging global warming and calling it a hoax; making spurious accusations that windmills cause cancer, energy efficient appliances are "worthless" and zero-emissions buildings "basically have no windows." At every turn and on every regulatory decision the administration embraces business over environmental interests.

"Biden wants to massively re-regulate the energy economy, rejoin the Paris climate accord, which would kill our energy totally, you would have to close 25 percent of your businesses and kill oil and gas development," Mr. Trump said on Wednesday as he announced a "top to bottom overhaul" of the National Environmental Policy Act, a bedrock environmental law since its passage in 1969. He offered no evidence to back up his statistics.

"When I think about climate change, the word I think of is ‘jobs,’ good-paying union jobs that will put Americans to work, making the air cleaner for our kids to breathe, restoring our crumbling roads, and bridges, and ports," Mr. Biden said on Tuesday as he outlined his plan.

The events captured the two candidates’ radically different beliefs about the global threat of the planet’s warming, and offered a glimpse of how they would lead a nation confronting a climate crisis over the next four years. For Mr. Trump, tackling global warming is a threat to the economy. For Mr. Biden, it’s an opportunity.

"They are polar opposites on almost everything to do with the environment but particularly climate change," said Christine Todd Whitman, the former Republican governor of New Jersey and administrator of the Environmental Protection Agency under George W. Bush.

Mr. Biden’s plan would spend $2 trillion over four years to put the United States on an "irreversible path" to net-zero emissions of planet-warming gases before 2050, meaning that carbon dioxide and other pollutants would be completely eliminated or offset by removal technology.

To do that, he called for clean energy standards that would achieve a carbon-free power sector by 2035; the energy efficiency upgrade of four million buildings in four years; and the construction of 500,000 electric vehicle charging stations. He also vowed to bring the United States back into the Paris Agreement, reinstate climate regulations that Mr. Trump has repealed and put more restrictions on things like emissions from vehicle tailpipes.

Mr. Trump has already moved to roll back virtually every effort the federal government made under President Barack Obama to combat climate change, from restricting emissions from power plants and vehicles to curbing methane from the oil and gas sector. He even rescinded an Obama-era executive order that urged federal agencies to take into account climate change and sea-level rise when rebuilding infrastructure.

The Trump administration’s latest overhaul to the National Environmental Policy Act highlighted their differences still more.

The changes finalized on Wednesday include a limit of two years to conduct exhaustive environmental reviews of infrastructure projects. They also revoked a requirement that agencies consider the cumulative environmental effects of projects, like their contribution to climate change.

Mr. Trump said the current lengthy process "has cost of trillions of dollars over the years for our country and delays like you wouldn’t believe."

In past campaigns, candidates have shied away from bold proposals on the environment, especially on climate change, but Patrick Murray, director of the Monmouth University Polling Institute, said that Mr. Biden has more political space to pursue his agenda amid the economic and public health crises plaguing the nation.

"With the pandemic shaking up the core of people’s lives, they are much less worried about bold and radical change right now," he said, "because that’s what they want in some way."

Those political dynamics have freed Mr. Biden to pursue policies that his allies hope will electrify younger, more liberal voters who were skeptical of him during the Democratic primaries, without automatically alienating more moderate voters, Mr. Murray said.

Certainly, Mr. Biden’s allies also see political risks, should he be perceived as moving too far left on issues like natural gas hydraulic fracturing, or fracking, a practice that is tied to many jobs in states like Pennsylvania. In contrast to a number of his Democratic primary opponents, he does not support a total ban on fracking.

"Fracking is not going to be on the chopping block," he said in an interview last week with WNEP-TV, an ABC affiliate that serves Northeastern and Central Pennsylvania.

In a call with reporters on Tuesday, Biden campaign officials stressed that his long-held view on the issue stands: "No new fracking on federal lands."

Mr. Trump is betting that his uncompromising, unchanging stands will appeal to business-minded voters and people who distrust government. But there is a risk to him, too.

Carlos Curbelo, a former Republican congressman from Florida who has championed a carbon tax to combat climate change, said he believes Mr. Trump’s disregard for the issue and his handling of the coronavirus are becoming linked to part of "the broader character question."

Such character questions resonate with "not just young voters who have rejected his stance on climate for quite some time but also middle-aged and older voters who now in the context of Covid prioritize leaders who are good crisis managers," he said.

Mr. Biden’s campaign criticized the president’s gutting of the environmental policy act as a way "to distract" from Mr. Trump’s failure to deliver an infrastructure plan. "He has failed to deliver any real plan to create jobs and instead is cutting corners to once again ignore science, experts, and communities and reservations entitled to clean air, water, and environments," read a campaign statement.

In some ways, the debate over climate reflects the broader political realignment in both parties that defined the 2016 campaign: working-class white voters, especially in rural areas, have moved farther from their union Democratic roots to embrace Mr. Trump and his energy policies, while educated, affluent white suburban voters, once staunchly Republican, drift toward the Democrats and appear increasingly open to more ambitious efforts to combat climate change.

"Biden’s pitch may play well with traditionally moderate Republican voters in the suburbs, just as Trump’s policy pronouncements may play well with traditionally more Democratic-leaning voters in other parts of the state, more rural parts of the state," said former Representative Ryan Costello, a Republican who represented the Philadelphia suburbs.

But pro-business voters may see the danger of a Biden victory as just as high, said Scott Jennings, a Republican strategist.

"A tremendous amount is at stake," he said. "Drastic overregulation and anti-business regulations could just decimate rural and middle-American economies already reeling."

Scientists said the next four years could be critical to whether greenhouse gas emissions from the United States rise or fall.

"We are on a trajectory to a hotter planet," said Waleed Abdalati, director of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder. Mr. Trump and Mr. Biden, he said, "represent two very divergent paths."