As President Obama moves forward with his plan to shield millions of undocumented immigrants from deportation, an existing program to protect young illegal migrants demonstrates the life-changing promise of executive action but also its profound shortcomings, according to experts and government documents.

The 2012 initiative has given temporary protection to slightly more than 700,000 people brought to the United States illegally as children. They say that program has helped them emerge from the shadows, making possible a work permit, a Social Security number and enhanced self-respect.

But hundreds of thousands who advocacy groups say are eligible have not applied under the program, known as Deferred Action for Childhood Arrivals, or DACA. Some immigrants say they are afraid they will be rejected and deported, while others are daunted by the $465 application fee and educational requirements. Still others remain unfamiliar with the program because of language and cultural barriers.

The DACA initiative is in many ways a template for the measures that Obama announced Nov. 20, which could give as many as 3.7 million undocumented parents of U.S. citizens and legal permanent residents relief from deportation for up to three years. And the record of the earlier program highlights both the potential and the pitfalls of Obama’s new undertaking.

Neither program has backing from Congress, and that limits their scope and durability. The next president could reverse either one with the stroke of a pen.

But people on all sides of the immigration debate point to DACA as a success, though a severely qualified one.

“We know it has made a tremendous beneficial impact in the lives of hundreds of thousands of people,’’ said Patrick Taurel, a legal fellow at the American Immigration Council, a pro-immigration group. “But it’s not enough. We ultimately need Congress to step in and create a permanent solution.’’

Although the rollout of DACA has been relatively smooth for a large government program put together in weeks, there have been problems. Immigration advocates say it can take a long time to process applications, and those who want to renew their status are facing a growing bottleneck.

Those difficulties have led some immigration experts to warn that the government could be overwhelmed by the even larger wave of applications expected under Obama’s latest executive action.

“DACA was a bit of a trial run, and overall it’s gone pretty well,’’ said David Martin, a University of Virginia law professor who was a senior legal official in the Department of Homeland Security. “But if we’re talking 4 million people with this new program, that’s going to be an even bigger challenge to gear up to handle it.’’

The same agency that administers DACA — U.S. Citizenship and Immigration Services, a part of the DHS — will oversee the new program, along with an expansion of DACA that could make up to 300,000 more young immigrants eligible.

A spokesman for the immigration agency said it is processing DACA applications, including renewals, in a timely fashion.

A sense of relief

Cesar Vargas embodies both the promise and the limits of DACA.

His mother and siblings crossed into the United States illegally from Mexico when he was 5. His father had recently died, making life a struggle at times. “Since then I have been a New Yorker, Brooklyn-raised,” said Vargas, 31.

Even before DACA, he graduated with a law degree from the City University of New York, set up his own lobbying firm and became an activist for young illegal migrants. Despite that, he couldn’t get a driver’s license or a credit card.

Qualifying for DACA brought an enormous sense of relief. “It really provided that strength to know I no longer could be deported any day,’’ he said.

Many of those granted DACA status say it has been a godsend. A survey of DACA recipients found that 60 percent had gotten a new job and 57 percent had obtained a driver’s license, according to the Migration Policy Institute, an advocacy group.

But like many of his fellow recipients, Vargas faces hurdles. Because he is still an illegal immigrant, with his deportation only temporarily deferred, he has been barred from practicing law. He is challenging that prohibition in a New York court.

Although DACA offers immigrants something closer to a normal life, there are critical things that it is not and was never intended to be. It does not grant immigrants legal status or citizenship or a pathway to either. And for many recipients, the benefits have proved bittersweet because their parents and other family members cannot apply.

“I was so excited because I really wanted to get a driver’s license and a Social Security number,” said Monica Camacho, 20, who came to the United States from Mexico in 2002. She obtained DACA status two years ago and is now a student at the Community College of Baltimore County in Essex, Md.

But Camacho said her parents and two older siblings remain illegal. “My dad has worked every day of his life, just for our future,” she said. “If you want to call my dad a criminal for wanting to give us a better future, it’s really unfair.”

Spreading the word

In neighborhoods throughout New York City, activists are passing out literature on street corners, making presentations at foreign consulates, and visiting churches and community centers — all in an effort to find people who are eligible for DACA but haven’t applied.

“We’re the cavalry of sorts, bringing DACA to all the different places where everyone else doesn’t have time to go,’’ said Betsy Plum, director of special projects for the New York Immigration Coalition.

“Fear is a huge barrier,’’ she said, adding that immigrants have trouble distinguishing between Citizenship and Immigration Services, which administers DACA, and U.S. Immigration and Customs Enforcement, which can initiate deportation. “There is a lot of mistrust of the government, and it’s really confusing — you have one government agency trying to help you and another trying to hurt you,’’ Plum said.

Hundreds of thousands who are eligible have not applied. Advocacy groups estimate that up to 1.2 million people were eligible for DACA when the program began and up to 2.1 million could qualify as they get older or go back to school. The Obama administration has never provided an official estimate.

Of the immigrants who have applied, 95 percent have received temporary relief from being deported.

A federal official said some of the people rejected for DACA have criminal records, while others failed to meet additional criteria, which include coming to the United States before their 16th birthday and living continuously in the United States since 2007.

Advocates say that instances of immigrants being deported after applying for DACA are rare. Former federal officials said they reassured immigration advocates from the start that the program would not be a pipeline for law enforcement.

Still, many undocumented immigrants won’t take the risk, according to Angelica Salas, executive director of the Coalition for Humane Immigrant Rights of Los Angeles.

“What you’re doing is turning yourself over [to the government]. ‘I’m here. I am telling you that I am undocumented. Here, let me prove it to you,’ ” she said.

A federal official involved in immigration policy said he understands the concern. “The next president can come in and wipe this all away, and then we have you registered,” the official said.

DACA’s education requirement also keeps some immigrants from applying. To qualify, they must have graduated from high school, have a GED or currently be in school. “If you need to enroll in an adult education program, that requires a substantial change to your daily routine,’’ Plum said.

Cultural and language barriers also play a role.

Nearly three-quarters of the DACA recipients were born in Mexico, but 19 other countries are represented, including several outside Latin America, notably South Korea, India and Poland. Some nationalities were hard to reach at first, advocates say, partly because the government took several months to translate materials into languages other than Spanish.

And in some cultures, especially Asian ones, it can be considered shameful to admit that you are an illegal immigrant, they said.

Up to the task

In the spring of 2012, immigration groups were clamoring for action by the Obama administration, and DACA was launched with great haste. Homeland Security officials had about two months to design and implement the program.

“We didn’t know the numbers of people who would apply. It could have been a hundred thousand, it could have been a million,’’ former homeland security secretary Janet Napolitano said in a recent interview. She added that Citizenship and Immigration Services “was concerned that they wouldn’t be able to handle it and that it would be an administrative failure.’’

Few are saying that now.

“It is an unquestioned success,’’ said John Sandweg, a former top adviser to Napolitano who helped design DACA. “There is no doubt that the work we did established that DHS is ready for immigration reform.’’

Others say that although the program has gone relatively smoothly, problems have emerged, raising questions about the immigration system’s capacity to handle a possible flood of applicants under the new program.

Taurel, of the American Immigration Council, said DACA applicants occasionally have to wait far longer than the six months DHS promises for a decision — and that renewal applications are being processed slowly even when they arrive early.

At the Consumer Law Legal Aid Justice Center in Falls Church, staff lawyer Simon Sandoval-Moshenberg said he is bracing for a crushing workload when the new wave of applications comes.

“I’m literally looking out the window into the Culmore neighborhood [of Fairfax County] and these apartment buildings,’’ he said, “and thinking, how in the world are we going to represent just these people in this one neighborhood, let alone all of Northern Virginia?”

Pamela Constable contributed to this report.