Jeh Charles Johnson was secretary of homeland security from December 2013 to January 2017.

President Trump must appoint a Senate-confirmed secretary of homeland security, and soon. Particularly in the current threat environment, our nation cannot afford a continued string of temporary, acting secretaries promoted from within the ranks of DHS to, as some would have it, simply receive and transmit orders from the White House. The job is one of the most complex and critical in the U.S. government.

If I were still in that office, two current threats would keep me up at night: the recent release of Islamic State fighters from captivity in Syria and the possibility that some may seek to travel here; and the ongoing Russian campaign to influence our elections. Public sources indicate the Russians are undeterred; the presidential election season is underway and, in many states, the cybersecurity of our election infrastructure is still a work in progress. The secretary of homeland security is the Cabinet-level official of our government principally responsible for confronting both these threats.

But under this president, the public and the media see the Department of Homeland Security largely as the instrument for hammering the administration’s hard-line views on immigration; they associate DHS with the ill-conceived travel ban in 2017, the failed and inhumane policy of separating families in 2018 and the crisis of more than 100,000 apprehensions a month along the southern border in the spring of 2019.

Homeland security is, in fact, much more — counterterrorism, cybersecurity, aviation security, maritime security, port security, the physical protection of our national leaders and U.S. government buildings, the detection of chemical, biological or nuclear threats to the homeland and the response to natural disasters.

The secretary of homeland security must be, in effect, the Cabinet-level sentinel, or — to use a phrase from the film “A Few Good Men” — the person we “need . . . on that wall,” constantly vigilant in monitoring threats from land, sea, air and in cyberspace. He or she has the ability to deploy and redeploy resources from across the vast homeland security bureaucracy to anticipate and meet those threats, including Customs and Border Protection (the largest single law enforcement agency of the government), Immigration and Customs Enforcement, the Transportation Security Administration, the Federal Emergency Management Agency, the Secret Service, the Coast Guard and the newly created Cybersecurity and Infrastructure Security Agency. The secretary of homeland security is also a member of the National Security Council, and is in a position to make important contributions there.

Trump says he likes senior political appointees in an “acting” capacity because it gives him “more flexibility” and he can “move so quickly.” This is the exact wrong attitude for a president to have toward his Cabinet. Though Cabinet officials serve at the pleasure of the president, they should provide much needed wisdom and advice that the president sometimes does not want to but needs to hear. Following Senate confirmation, they generally serve for the duration of the president’s four-year term, and engage in the longer-term strategic thinking necessary to run a large government agency. When an acting official feels obliged to audition day to day for a president who quickly sours on subordinates and does not like to hear views contrary to his own, the consequences can be disastrous.

The large and demanding job of secretary of homeland security warrants a qualified and experienced person, carefully selected by the president with the advice and consent of the Senate; the current threat picture demands it.

I urge that, in his search for a new secretary, President Trump select a person of wide experience and broad stature. Despite the sharp political differences, when President-elect Trump announced John Kelly’s appointment as my successor, I called Mr. Trump personally to congratulate him on the fine choice, and told the DHS workforce to greet and embrace Gen. Kelly as a man of character and integrity. Secretary Kelly then had the job for all of six months.

The president should resist the temptation to nominate a pronounced hard-liner on immigration who will be a lightning rod to lead a Cabinet department already in the thick of political storms. I believe there are well-qualified and patriotic individuals such as Kelly and former defense secretary Jim Mattis who, when called upon by the commander in chief to serve, will serve. The possibility that such an individual’s views and perspectives do not align perfectly with the president’s is a virtue in a Cabinet, not a vice.

Fill the job with someone well qualified, and fill it soon, Mr. President. A president who leaves the job vacant for too long is neglecting his own duty to defend the homeland and keep the American people safe.

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