Mr. Schumer told his staff that the proposal was a nonstarter, and he directed them to quickly spread the word that Democrats would oppose the bill as it was, according to several people involved in the discussions who, like more than dozen lawmakers and senior officials interviewed for this story, spoke on the condition of anonymity to describe the roller coaster negotiations that led to the passage of the largest stimulus measure in modern American history.

Then he called his colleagues — including Senator Elizabeth Warren of Massachusetts, a former presidential candidate who is influential on the left — to alert them to what he saw as a Republican ploy to muscle through a corporate giveaway. Within hours, Ms. Warren declared on Twitter that she would oppose the “giant slush fund” and urged other Democrats to join her. On a conference call later that day, Senator Patty Murray of Washington, not known for a temper, said she would be “a damn no” on the bill and urged her colleagues to do the same, which they did unanimously in a vote that sent futures markets plummeting.

It was a shocking and politically perilous decision in the middle of a paralyzing national crisis, a moment when lawmakers are traditionally expected to put aside differences for the good of the country, or face a political backlash.

The move was particularly infuriating for Republicans who had been willing to momentarily abandon their small-government zeal and agree to large additions to safety-net programs, including direct payments to Americans and a substantial increase in jobless aid, in the interest of sealing a quick deal with Democrats. Senator Mitch McConnell, Republican of Kentucky and the majority leader, would later call it a “stupid vote,” but Democrats said it proved crucial.

“It showed McConnell that he was going to have to deal with us,” Mr. Schumer said.

The moment was a turning point for the rapid and fitful negotiations over the stimulus measure, which came together over a handful of frenzied days on Capitol Hill, as global markets convulsed with worry and lawmakers scrambled to agree before Covid-19 could infect their ranks and cripple Congress.