WASHINGTON — President Donald Trump on Tuesday began dismantling the government program protecting hundreds of thousands of young immigrants who were brought into the country illegally as children. Attorney General Jeff Sessions declared the Obama administration’s program “an unconstitutional exercise of authority” that must be revoked.

New applications will be halted for President Barack Obama’s Deferred Action for Childhood Arrivals program, which has provided nearly 800,000 young immigrants a reprieve from deportation and the ability to work legally in the U.S. in the form of two-year, renewable work permits.

“I’m here today to announce that the program known as DACA that was effectuated under the Obama administration is being rescinded,” Sessions announced.

But the administration is giving Congress six months to come up with a legislative fix — “should it choose to,” Sessions said — before the government stops renewing permits for people already covered by the program.

According to Department of Homeland Security officials, people with permits whose renewals are set to expire between now and March 5, 2018, will be able to re-apply — so long as their applications are submitted by Oct. 5, 2017, one month from Tuesday. No permits will be revoked before their existing expiration dates, and applications already in the pipeline will be processed, they said.

Trump, in a statement, said the change would be “a gradual process, not a sudden phase out.”

“Thus, in effect, I am not going to just cut DACA off, but rather provide a window of opportunity for Congress to finally act,” he said. He said he did not favor punishing children for the actions of their parents. At the same time, though, “we must also recognize that we are a nation of opportunity because we are a nation of law” and “young Americans have dreams, too.”

His action drew swift criticism from many immigration advocates and Democratic lawmakers.