THROUGH NO FAULT of their own, a generation of children of undocumented immigrants who came to the United States as young children has lived in the shadows of American life. President Obama’s announcement Friday that the government will give them a two-year reprieve from possible deportation will ease fears that long have haunted many of them. Those who take advantage of the offer and are approved can then seek official work permits.

The reprieve is aimed at those who might be described as most likely to succeed: an estimated 800,000 people, under 30 years old, in school or having graduated or served in the armed forces, without a significant criminal record, who pose no threat to national security or public safety. In many cases they are strivers who have known only America as home. Deporting them to countries they left as children makes no sense.

So on policy grounds, the action is right; we have strongly supported legislation that would accomplish a similar purpose. But there’s the rub. Congress chose not to act on that legislation. In December 2010 the Dream Act, which would have created a pathway toward citizenship for young, undocumented immigrants, died in the Senate. Mr. Obama’s announcement does not go as far as that bill; it does not provide a path to citizenship. Instead the president is choosing to selectively enforce existing law. Is this how our system is supposed to work?

Janet Napolitano, secretary of homeland security, said that the government determined that prosecution of these young, undocumented immigrants is a low priority compared to pursuing other cases of criminals or people who pose a risk to national security. The immigration laws must be enforced, the secretary insisted, but “they are not designed to be blindly enforced.” What does that mean? Sure, prosecutors always have to make choices and set priorities. But it is unsettling to see the president and his administration openly declare that they will not enforce a law on the books because they don’t think it’s a good law. The obvious political ramifications so close to an election, with Mr. Obama hoping to attract Latino votes, only heighten the unease.

Mr. Obama’s announcement offers a temporary reprieve to these young immigrants, but after the celebrations, another worry looms. While no longer threatened with deportation, they could find themselves still in a twilight zone, an underclass without a map toward a full role in American society.

For the 11 million undocumented immigrants who are in the United States today, as well as for the nation’s economy and law enforcement system, the best and only sustainable solution is a comprehensive overhaul of the law.

The president’s action should not be another excuse for bickering and delay.