Border Patrol Agent Eduardo Olmos walks near the secondary fence separating Tijuana, Mexico, background, and San Diego in June 2016. (Gregory Bull/AP)

President Trump’s plan for an aggressive hiring surge of 15,000 Border Patrol and immigration personnel to help keep out undocumented immigrants is unrealistic — and the Department of Homeland Security has not made a case for it, the agency’s watchdog says.

A report released this week by the DHS inspector general concludes that based on its rigorous screening requirement for law enforcement jobs and the relatively high rate of attrition among Border Patrol agents, Homeland Security would have to vet 750,000 applicants to find 5,000 qualified personnel.

In addition, to hire the 10,000 Immigration and Customs Enforcement (ICE) agents the president called for in executive orders he issued in his first days in office, a pool of 500,000 candidates would need to apply, auditors found.

The report calls into question whether DHS officials even need 15,000 new hires to target undocumented immigrants. Agency leaders have done such poor planning for what their workforce should look like, with an understaffed, poorly trained human resources operation, that they cannot justify thousands of new employees, the report says.

“Neither [U.S. Customs and Border Protection] nor [ICE] could provide complete data to support the operational need or deployment strategies for the additional 15,000 agents and officers,” the report by the office of Inspector General John Roth said.

DHS officials told auditors that they are still three to four years from getting a system in place that will be able to tell them how many new personnel they need and where to deploy them.

“Without comprehensive staffing models, operational needs analyses, and deployment strategies, CBP and ICE will not be able to identify clearly the correct number and type of employees required, what positions must be filled, or where to deploy those employees,” the report said.

Trump promised on the campaign trail to vastly beef up enforcement against undocumented immigrants with a new border wall, a surge in agents to help seal off the Southwest border with Mexico and a “deportation force” to arrest people in the country illegally. The 15,000 new front-line employees would come with almost 9,600 more technical and support staff, the report said.

The White House has stalled in getting Congress to approve funding to start construction of a wall. The hiring effort has stalled, too — even though the Office of Personnel Management gave Homeland Security officials special authority for a year to fill jobs without competition, circumventing the government’s cumbersome hiring rules.

The immigration and Border Patrol departments have long had trouble hiring qualified candidates quickly and retaining them because of polygraph requirements, remote workplaces and low pay compared with other law enforcement jobs. Turnover averages 6 percent a year for the Border Patrol and 4 percent for ICE.

Homeland Security’s personnel office suffers from “fragmented systems, duplicative and paper-based processes” and not enough of the staff or automation it needs to hire this many new employees, auditors found.

They note that 61 percent of the 21,000 Border Patrol agents actually patrol the Southwest border, a number that “may actually understate” the number of employees assigned to management and administrative work. Almost 1,000 more agents than headquarters officials have allocated are assigned to intelligence roles in the field, auditors found.

“The use of Border Patrol Agents performing duties not directly tied to ‘ensuring complete operational control of the border’ calls into question the department’s operational need for 5,000 new agents,” the report said.

It also questions whether DHS even needs 15,000 new officers to carry out its mission as immigration arrests decline. The Border Patrol apprehended 12,193 people along the Southwest border in March compared with 18,754 in February, a 35 percent decrease, and 31,577 in January, a 61 percent slide.

“These trends may impact the number of new Border Patrol agents needed,” the report said.

In its response to auditors, a DHS official defended the president’s mandate to hire thousands of new agents, calling the push a response to “emerging operational needs and changes in technology.”

“DHS remains committed to ensuring correct staffing levels, ratios and placements,” Jim H. Crumpacker, the department’s liaison with the inspector general, wrote.

Congressional Democrats pounced on the findings as proof that the president rushed to fulfill a campaign promise without planning.

“It’s not surprising that President Trump’s decree to drastically expand the Border Patrol and increase his deportation force was put in place without a clear need for the 15,000 new agents and officers or a plan for hiring and deploying them,” Rep. Bennie Thompson (Miss.), the ranking Democrat on the House Homeland Security Committee, said in a statement.

“I hope DHS goes back to the drawing board and shows us the need for new personnel along with a clear plan on how to better manage them once deployed.”