Saying it had the right to block "controversial or unsavory" text messages, Verizon Wireless last week rejected a request from Naral Pro-Choice America, the abortion rights group, to make Verizon's mobile network available for a text-message program.
But the company reversed course this morning, saying it had made a mistake.
"The decision to not allow text messaging on an important, though sensitive, public policy issue was incorrect, and we have fixed the process that led to this isolated incident," Jeffrey Nelson, a company spokesman, said in a statement.
"It was an incorrect interpretation of a dusty internal policy," Mr. Nelson said. "That policy, developed before text messaging protections such as spam filters adequately protected customers from unwanted messages, was designed to ward against communications such as anonymous hate messaging and adult materials sent to children."
Mr. Nelson noted that text messaging is "harnessed by organizations and individuals communicating their diverse opinions about issues and topics" and said Verizon has "great respect for this free flow of ideas."
The other leading wireless carriers had accepted the Naral program, which allows people to sign up for text messages from Naral by sending a message to a five-digit number known as a short code.
Text messaging is a growing political tool in the United States and a dominant one abroad, and such sign-up programs are used by many political candidates and advocacy groups to send updates to supporters.
But legal experts said private companies like Verizon probably have the legal right to decide which messages to carry. The laws that forbid common carriers from interfering with voice transmissions on ordinary phone lines do not apply to text messages.
In reversing course today, Verizon did not disclaim the power to block messages it deemed inappropriate.
The dispute over the Naral messages was a skirmish in the larger battle over the question of "net neutrality" - whether carriers or Internet service providers should have a voice in the content they provide to customers.
"This is right at the heart of the problem," said Susan Crawford, a visiting professor at the University of Michigan law school, referring to the treatment of text messages. "The fact that wireless companies can choose to discriminate is very troubling."
In initially turning down the program, Verizon, one of the nation's two largest wireless carriers, had told Naral that it does not accept programs from any group "that seeks to promote an agenda or distribute content that, in its discretion, may be seen as controversial or unsavory to any of our users." Naral provided copies of its communications with Verizon to The New York Times.
Nancy Keenan, Naral's president, said Verizon's initial decision interfered with political speech and activism.
"No company should be allowed to censor the message we want to send to people who have asked us to send it to them," Ms. Keenan said. "Regardless of people's political views, Verizon customers should decide what action to take on their phones. Why does Verizon get to make that choice for them?"
On Wednesday, Mr. Nelson, the Verizon spokesman, said the initial decision had turned on the subject matter of the messages and not on Naral's position on abortion. "Our internal policy is in fact neutral on the position," Mr. Nelson said. "It is the topic itself" - abortion - "that has been on our list."
Naral provided an example of a recent text message that it has sent to supporters: "End Bush's global gag rule against birth control for world's poorest women! Call Congress. (202) 224-3121. Thnx! Naral Text4Choice."
Messages urging political action are generally thought to be at the heart of what the First Amendment protects. But the First Amendment limits government power, not that of private companies like Verizon.
In rejecting the Naral program, Verizon appeared to be acting against its economic interests. It would have received a small fee to set up the program and additional fees for messages sent and received.
Text messaging programs based on five- and six-digit short codes are a popular way to receive updates on news, sports, weather and entertainment. Several of the leading Democratic presidential candidates have used them, as have the Republican National Committee, Save Darfur and Amnesty International.
Most of the candidates and advocacy groups that use text message programs are liberal, which may reflect the demographics of the technology's users and developers. A spokeswoman for the National Right to Life Committee, which is in some ways Naral's anti-abortion counterpart, said, for instance, that it has not dabbled in text messaging.
Texting has proved to be an extraordinarily effective political tool. According to a study released this month by researchers at Princeton and the University of Michigan, young people who received text messages reminding them to vote in November 2006 were more likely to go to the polls. The cost per vote generated, the study said, was much smaller than other sorts of get-out-the-vote efforts.
Around the world, the phenomenon is even bigger.
"Even as dramatic as the adoption of text messaging for political communication has been in the United States, we've been quite slow compared to the rest of the world," said James E. Katz, the director of the Center for Mobile Communication Studies at Rutgers University. "It's important in political campaigns and political protests, and it has affected the outcomes of elections."
Timothy Wu, a law professor at Columbia, said it was possible to find analogies to Verizon's decision abroad. "Another entity that controls mass text messages is the Chinese government," Professor Wu said.
Jed Alpert, the chief executive officer of Mobile Commons, which says it is the largest provider of mobile services to political and advocacy groups, including Naral, said he had never seen a decision like Verizon's.
"This is something we haven't encountered before, that is very surprising and that we're concerned about," Mr. Alpert said.
Professor Wu pointed to a historical analogy. In the 19th century, he said, Western Union, the telegraph company, engaged in discrimination, based on the political views of people who sought to send telegrams. "One of the eventual reactions was the common carrier rule," Professor Wu said, which required telegraph and then phone companies to accept communications from all speakers on all topics.
Some scholars said such a rule was not needed for text messages because market competition was sufficient to ensure robust political debate.
"Instead of having the government get in the game of regulating who can carry what, I would get in the game of promoting as many options as possible," said Christopher S. Yoo, a law professor at the University of Pennsylvania. "You might find text-messaging companies competing on their openness policies."
© 2007 The New York Times Company