Apple declined to comment on the letter.

The questions coming from Washington cut to the heart of today’s smartphone competition. Rival smartphone makers credit iMessage with helping Apple expand its smartphone market share in the United States to more than 50 percent of smartphones sold, up from 41 percent in 2018, according to Counterpoint Research, a technology firm.

Messaging has been a key part of Apple’s strategy to sell more iPhones. For years, it has made exchanges between iPhones and Android devices as basic as the texts between decades-old flip phones. Texts between iPhone users appear in blue and can be tapped to give a thumbs up, but texts from Android users appear in green and have no simple perks.

Android companies have tried to fight back. An Android smartphone maker, Nothing, has collaborated with an app called Sunbird to offer iMessage. Google, which created the Android operating system, has pressured Apple to adopt a technology called rich communication services, which would make it possible to send high-resolution video and images between competing smartphones.

But their efforts have not made much of a dent. Last month, Apple said it would adopt the technology in the coming year. The move means Android users will enjoy benefits like sharing higher-resolution videos but be stuck with the green bubbles for text messages, which have become stigmatized and associated with less wealth.

“Everyone is watching to see what kind of response Apple is going to have to Beeper Mini,” said Cory Doctorow, a special adviser to the digital rights advocacy group Electronic Frontier Foundation who has written a book about interoperability across different technologies. “We can’t tell how worried they are internally, but their response could have a huge impact on how messaging works.”