Trump’s plan to get tough on immigration faces legal and logistical constraints.

WASHINGTON — President Donald Trump’s efforts to secure the nation’s borders and get tough on immigrants in the country illegally, announced just days after he took office, face serious logistical problems along with the legal challenges that threaten his ability to make good on a central campaign promise.

The crackdown requires a vast commitment of resources, including hiring 15,000 new border patrol and immigration enforcement agents, which officials say will take at least two years to accomplish.

Large detention centers for thousands of Central American asylum seekers who cross the southern border will need to be built because of an executive order by Trump calling for an end to “catch and release” — the Obama administration policy that the immigrants be released temporarily into the United States while their cases are processed.

In the meantime, the White House has not produced a replacement for another executive order by Trump, a ban on travel from seven predominantly Muslim countries that was blocked by a federal court. The president said Thursday that to withstand the legal challenges, his lawyers are preparing a more narrow executive order that is likely to exempt green card holders, students, tech workers and those with long-standing connections to the U.S.

In a rambling news conference Thursday, the president said his administration had undertaken “the most substantial border security measures in a generation,” and he said that efforts to find and deport “criminal aliens” would make the U.S. safer.

“Some people are so surprised that we’re having strong borders,” Trump said. “Well, that’s what I’ve been talking about for a year and a half, strong borders. They’re so surprised: ‘Oh, he is having strong borders.’ Well, that’s what I’ve been talking about to the press and to everybody else.”

But his early efforts to translate all of that talk into action are running into the reality of governing in Washington, where legal constraints on taking action — and debates about paying for it — are legendary.

Trump has promised to hire 15,000 new Border Patrol and Immigration and Customs Enforcement agents as part of a larger deportation force that can remove millions of immigrants in the country illegally, something he repeatedly promised to do during the campaign.

But hiring such a large number of agents in a short period of time would be nearly impossible, according to John F. Kelly, the former general whom Trump chose to be the secretary of the Department of Homeland Security.

“I don’t believe we’re going to get 10,000 and 5,000 on board within the next couple of years,” Kelly told lawmakers on Capitol Hill this month, explaining that stringent hiring standards and training regiments slow down the process.

“I’d rather have fewer and make sure that they’re high-quality people that are already serving in those organizations, already well trained, but I will not skimp on the training and the standards,” Kelly said.

One of the problems that Kelly faces is a polygraph test that prospective agents, including those seeking to work for the Border Patrol, must take. According to a former senior homeland security official, nearly 60 percent of applicants fail it.

The test was first put in place after another surge in hiring during the George W. Bush administration. Thousands of people were hired without being properly vetted, which resulted in dozens of corruption cases involving Border Patrol and other agents, who were accused of taking bribes and providing information to Mexican drug cartels.

It is unclear how Trump and Kelly plan to solve the manpower problem.

One option, swiftly denied Friday by the White House, was a plan to use as many as 100,000 National Guard troops as part of a nationwide deportation force that would help to augment federal agents and local authorities newly deputized to enforce the nation’s immigration laws.

The idea emerged in a draft memorandum, first reported by The Associated Press, which asserted that National Guard troops, under the direction of governors in border states, are “particularly well suited to assist in the enforcement of federal immigration law and augment border security operations by Department components.”

Gillian M. Christensen, acting press secretary for the Department of Homeland Security, said the memorandum was a “very early, pre-decisional draft that never made it to the secretary and was never seriously considered by the department.” Sean Spicer, White House press secretary, said that the AP report was “100 percent not true.”

But advocates for immigrants reacted with alarm.

“The administration wants to put on a show,” said Kevin Appleby, senior director of international migration policy at the Center for Migration Studies of New York. “Their intent is to create fear, to create an environment in which people either self-deport or hide in the shadows.”