Khan hasn’t won a case against big tech---cred is zero and the da is thumped.

For too long, Ms. Khan said at the time, the agency had been a weak cop and needed to challenge behemoths like Microsoft, Amazon, Meta and Google in the courts to stem their growing power. Even if the F.T.C. lost the cases, she later added, they would be a partial victory because the agency would signal that antitrust laws needed to be updated for the modern internet era.

But on Tuesday, Ms. Khan suffered the biggest blow yet to her hallmark agenda. A federal judge rejected the F.T.C.’s attempt to stop Microsoft’s $70 billion acquisition of the video game maker Activision Blizzard from closing, saying the agency failed to prove the deal would reduce competition and harm consumers. On Wednesday, the F.T.C. filed a notice that it would appeal the judge’s decision.

That followed a loss in February, when a judge rejected an F.T.C. lawsuit seeking to block Meta from buying the virtual reality start-up Within.

The defeats raise questions about Ms. Khan’s ability to carry out her ambitious goal of reversing decades of weak antitrust enforcement, as political pressure mounts and patience wanes for the 34-year-old academic, who has ruffled the feathers of corporate America. Ms. Khan’s critics are more emboldened and are speaking out more loudly to poke holes in her take-it-to-the-courts strategy, saying the losses are not even partial wins — they’re just losses.

“I completely disagree with this approach,” Anthony Sabino, a professor of business and law at St. John’s University, said of Ms. Khan’s methods. “She’s trying to change a century’s worth of antitrust law overnight, and that’s not necessarily wise.”

Adam Kovacevich, the chief executive of Chamber of Progress, a tech trade group, said the defeats made the F.T.C. appear less credible. “All these court losses are making their threats look more like a paper tiger,” he said.

Others wondered if Ms. Khan was wasting the F.T.C.’s resources on can’t-win cases. “They’ve crossed the line to being reckless with the cases they are bringing,” said Ashley Baker, a director of public policy for Committee for Justice, a conservative think tank.