President Biden on Tuesday called denying the effects of climate change “really, really dumb” and said that extreme heat and other weather disasters fueled by rising global temperatures have cost billions of dollars and thousands of American lives.

“Ignoring climate change is deadly and dangerous and irresponsible,” Mr. Biden said. He warned that temperatures have already shattered records this summer, and are expected to climb, as he proposed new protections for workers exposed to dangerous heat on the job.

Mr. Biden spoke at the District of Columbia’s emergency operations center, where he and cabinet officials were briefed on extreme weather developments across the country. His remarks were some of the most forceful he’s given on climate change, and he used the moment to denounce Republican lawmakers who oppose his policies.

“How can you deny climate change for God’s sake? Mr. Biden said. “They must be living in a hole somewhere.”

“Anyone who willfully denies the impacts of climate change is condemning the American people to a dangerous future and either is really, really dumb or has some other motive,” he said.

The proposed regulation from the Occupational Safety and Health Administration comes as a heat wave settles over California this week and triple-digit heat is also expected to scorch Portland, Ore., over the July 4 weekend. That follows an early heat wave last month that had much of the country sweltering.

Mr. Biden noted that extreme heat is the deadliest weather-related hazard in the United States, responsible for more deaths than floods, hurricanes and tornadoes combined. Climate change, driven by the burning of oil, gas and coal, is causing heat waves to last longer. Last year, the hottest in recorded history, brought 2,300 heat-related fatalities in the United States, most likely an undercount because heat exacerbates other health conditions, while emergency room visits for heat illnesses surged.

The regulation, if implemented, would for the first time require employers to monitor workers and provide rest areas and water when the heat index reaches 80 degrees or higher. Employers would also be required to establish heat safety plans. The rule could apply to about 35 million Americans who work both indoors and outdoors.

The heat index accounts for both air temperature and humidity.

Mr. Biden also announced nearly $1 billion from the bipartisan infrastructure law to help communities build resilience to weather-related disasters.

Environmental groups and labor unions praised the proposed regulation. Liz Shuler, president of the A.F.L.-C.I.O., said in a statement the heat worker protections “could not be more urgently needed.”

She called on Congress to codify the new protections into law.

Aru Shiney-Ajay, executive director of the Sunrise Movement, a youth-oriented climate group, called the protections “a huge movement win and the kind of action young people are looking for from President Biden.”

The presidential election this year will have significant consequences for the implementation of the workplace rule, which could be discarded if Mr. Trump were to win the White House in November. Mr. Trump has pledged to roll back Mr. Biden’s entire environmental agenda, and routinely mocks both the science and effects of climate change.

The Supreme Court also unleashed a wave of decisions over the past week that combine to drastically diminish the authority of federal agencies to regulate industries and the workplace. The decisions are expected to make it easier for businesses and industries to get federal regulations overturned.

The proposed heat rule is already facing opposition from industry groups that say it would drive up costs and would be cumbersome to administer. Opponents have indicated they intend to challenge the legality of the rule as soon as it is implemented, possibly late this year.

Jonathan Berry, who served as a senior Labor Department official under Mr. Trump, said the combined Supreme Court decisions “are clearly expressive of a judicial mood that would be very hostile to this rule making.”

Marc Freedman, a vice president at the U.S. Chamber of Commerce, the country’s largest business lobbying group, wrote that such a rule would present huge challenges for employers and that “it is extraordinarily difficult for them to determine when heat presents a hazard because each employee experiences heat differently.”

Gov. Ron DeSantis of Florida and Gov. Greg Abbott of Texas, both Republicans, have signed legislation to prevent local governments from requiring heat protections for outdoor workers.

It was one of Mr. Biden’s first events since the presidential debate on Thursday, where his shaky performance raised questions about his age, and he used the moment to go on the attack. The 81-year-old president and his campaign have been working to reassure donors that he can beat former President Donald J. Trump, the presumed Republican nominee, and serve another four years in the White House.

Douglas Brinkley, a presidential historian at Rice University who focuses on environmental policy, said by combining climate change and protections for workers, Mr. Biden was going to his “comfort place” for an easy political win.

“He likes being ‘lunchbox Joe’ and being on the side of labor and the working class,” Mr. Brinkley said. “It’s a good issue for Joe Biden. It reminds environmentalists that he’s an ally, and rings the climate bell, but also shows he has the back of and the health of the labor movement.”

Heat waves last longer now than they did decades ago, according to an Environmental Protection Agency report also expected to be released on Tuesday. In recent years heat waves in major U.S. urban areas have lasted, on average, about four days, a full day longer than the average in the 1960s, it found.

The proposed workplace rule incorporates two so-called heat triggers, with each threshold requiring employers to implement different protections for workers. An initial trigger would be set at a heat index of 80 degrees Fahrenheit and would require that employers offer access to drinking water, break areas where workers can cool off and rest breaks when needed.

Workers would also be entitled to so-called acclimatization plans, which would allow newer employers who are not as adjusted to extreme heat to gradually increase their hours as they adapt to conditions.

A second trigger, set at a heat index of 90 degrees Fahrenheit, would require rest breaks of 15 minutes every two hours and would mandate that employers monitor their workers for symptoms of heat illness.

Employers who fail to meet the new standards could face fines of more than $16,000 depending on the violation, a senior administration official said.

In addition to the impact on health, heat takes an economic toll. The California state insurance commissioner released a report this week finding that heat waves had cost Californians at least $7.7 billion over the last decade and had killed nearly 460 people. In 2021, more than 2.5 billion hours of labor in the U.S. agriculture, construction, manufacturing and service sectors were lost to heat exposure, according to data compiled by The Lancet. Another report found that in 2020, the loss of labor as a result of heat exposure cost the economy about $100 billion, a figure projected to grow to $500 billion annually by 2050.