The regulators’ moves are small and preliminary, and could easily come to nothing. But if the agencies pursue cases, Google and Amazon will almost certainly face reams of bad publicity, rising consumer distrust and falling employee morale. An inquiry would remind everyone that Google, with its early motto of “Don’t be evil,” held itself to standards it sometimes could not match.

“This is more of a warning to the companies that they’re being carefully scrutinized and they need to be careful not to play fast and loose given their dominant positions in the digital marketplace,” said Gene Kimmelman, a former senior antitrust official at the Justice Department who is now president of the consumer group Public Knowledge.

A prospect that should really worry Google and Amazon is a replay of the government’s case against Microsoft in the 1990s. Microsoft did not have to break itself into two, which was the government’s goal. But the company was distracted for at least a decade, which allowed space for start-ups like Google. Microsoft’s reputation took a dive.

“The damage to the monopolist’s position comes from the public airing of the facts,” said Gary Reback, a Silicon Valley lawyer who was instrumental in the case against Microsoft and has worked with companies that argue they have suffered unfair competition from Google.

Even without formal government investigations, the political pressure for action has been mounting.

“It’s time to fight back,” Senator Elizabeth Warren of Massachusetts, a Democratic candidate for president, said after news of the Google developments emerged. Senators Josh Hawley, Republican of Missouri, and Richard Blumenthal, Democrat of Connecticut, each said the scrutiny of Google was overdue.