The $369 billion climate and tax package forged in a surprise deal by Senate Democrats on Wednesday would be the most ambitious action ever taken by the United States to try to stop the planet from catastrophically overheating.

The agreement, which Senate Democrats hope to pass as early as next week, shocked even some who had been involved in the sputtering negotiations over climate legislation during the past year. The announcement of a deal, after many activists had given up hope, almost instantly reset the role of the United States in the global effort to fight climate change.

And it was delivered by Senator Joe Manchin III of West Virginia, the holdout Democrat who had been reviled by environmentalists and some of his own colleagues after he said this month that he could not support a climate bill because of inflation concerns.

“By a wide margin, this legislation will be the greatest pro-climate legislation that has ever been passed by Congress,” Senator Chuck Schumer, the Democratic majority leader, said in announcing the deal with Mr. Manchin.