The scrutiny is set to intensify when the Senate Banking Committee holds a hearing on Libra on Tuesday. The House Financial Services Committee is scheduled to hold a separate hearing about the project a day later.

The top Democrat on the Senate Banking Committee, Sherrod Brown of Ohio, sent a letter on Wednesday to Mr. Powell and others at the Fed, asking the central bank to protect consumers and the economy from “Facebook’s Monopoly Money.”

“We cannot allow giant companies to assert their power over critical public infrastructure,” the letter said.

The blowback to Libra is the latest challenge for Facebook, which has been under fire for not guarding its users’ data privacy and for spreading disinformation and misinformation globally. Those issues have hurt the Silicon Valley company’s once stellar reputation and raised questions about whether it can properly exercise its power. Since then, Senator Elizabeth Warren, the Massachusetts Democrat who is running for president, has called for Facebook and other tech companies to be broken up and the Federal Trade Commission has agreed to handle potential antitrust investigations into the social network.

A Facebook spokeswoman pointed to a letter that David Marcus, who is overseeing the project, sent this week to members of Congress who had criticized Libra. In the letter, Mr. Marcus wrote that “policymakers and others are raising important questions.”