“I still think we can find the revenues to do the $3.5 trillion,” Sen. Mark Warner of Virginia, a Democrat on the Senate Finance Committee, said of his party’s sprawling spending package. “If we cannot generate the revenue, it all becomes somewhat moot.”

He added, “It’s not going to be easy.”

Democrats’ ability to roll back tax breaks for corporations and individuals will ultimately determine how much they’ll be able to spend on their bill to boost the child tax credit, provide paid family and sick leave and expand health care programs over the next decade as many moderate Democrats have argued that most of — if not all of — those programs need to be fully paid for.

Democratic leaders and top committee chairs have been bracing for the moment.