As lawmakers last week completed a record-shattering economic-rescue package estimated at $2 trillion, Senate Minority Leader Chuck Schumer (D., N.Y.) predicted: “This is certainly not the end of our work here in Congress—rather the end of the beginning.”

Legislators from both parties, administration officials, economists, think tanks and lobbyists are already roughing out the contours of yet another emergency-spending package—perhaps larger than the last—to try to keep the coronavirus crisis from turning into a 21st-century...