Homeland Security Secretary Jeh Johnson has assailed Donald Trump’s vow to build a massive wall along the U.S.-Mexico frontier as “overheated, oversimplistic rhetoric” and a bad idea.

He’s called Trump’s controversial remarks toward Muslims – and similar words from Ted Cruz before the Texas senator’s exit from the presidential race – “beyond the pale” and said anti-Muslim rhetoric burns bridges instead of building them.

But now, on the subject of the presumptive GOP nominee for president and how the agency tasked with border security would actually build a wall to keep out illegal Mexican migrants, Johnson is more or less .. mum.

“I’ve talked in public settings about the wall that exists, now, on the border,” he said in an interview in his office this week. “I’ve said that we’ve got fences in places where it makes sense to build fences.”

Johnson pulled from his desk drawer a print-out of a PowerPoint presentation on the existing fence that now stretches about 650 miles along the Southwest border (the entire border is almost 2,000 miles). And he described how the fence is only one feature of a broader border strategy that includes expanded sensors, drones and other technology.

But the homeland chief did not want to talk about Trump or his controversial immigration plan.

With the presidential election six months away, Johnson and other top Obama administration officials must be careful what they say about the presidential race because of a law called the Hatch Act that prohibits federal employees from engaging in political activity on the job.

Lawyers from the Office of Special Counsel, the small federal agency that prosecutes violations of the Hatch Act, are starting to meet with employees in federal agencies across government to reinforce the rules for everyone – from administrative assistants to Cabinet secretaries.

While in recent months Johnson has avoided referring to Trump (or Cruz) by name, sticking instead to their policies, the Obama administration has strictly enforcing the Hatch Act.

This week, the special counsel staff took its expertise to Homeland Security headquarters, where the lawyers held a training session with dozens of employees, officials with the special counsel’s office said.

“We’re trying to reach out across the federal government at all levels to remind people what they can and cannot do,” agency spokesman Nick Schwellenbach said.

The Hatch Act prohibits federal employees at all levels, whether civil servants or political appointees, from political activity while they’re on duty or using their official authority.

They can make partisan remarks when speaking in what’s called their personal capacity off duty, but not when they’re speaking about agency business in an official role.

The rules don’t apply to President Obama and Vice President Biden, who are free to say what they want about the race to succeed them.

A small subset of political appointees, including Cabinet secretaries, at the Justice, Defense, Homeland Security and State departments have further restrictions, and cannot, for example, work on a candidate’s behalf in their free time.

The special counsel’s office found that former health and human services secretary Kathleen Sebelius violated the Hatch Act in 2012 when she publicly endorsed President Obama’s reelection at a taxpayer-funded, public event.

Johnson said he looks forward to briefing the transition staffs for Trump and Hillary Clinton, the likely Democratic nominee, when they assemble after the nominating conventions this summer to get briefings from federal agencies on a smooth handover of power, whoever wins in November.

“I’ll have plenty of advice for them about Homeland Security,” Johnson said. “I would just rather not forecast it publicly.”

(c) 2016, The Washington Post · Lisa Rein