New Jersey Gov. Chris Christie endorsed Republican presidential frontrunner Donald Trump for president on Friday. Here's what the two have said about each other in the past. (Victoria Walker/The Washington Post)

I have probably interviewed New Jersey Gov. Chris Christie more times for more hours than any journalist outside New Jersey. I’ve known him since he started running for governor in 2008.  You can understand my shock then when a man who claimed to be serious about public service, who ran on concrete policies and a serious national security platform and who seemed genuinely concerned about an unqualified person becoming president would embrace a know-nothing buffoon like Donald Trump.

Whether damning Trump for ignorance about Vladimir Putinridiculing Trump’s idea to ban Muslims coming into the United States or disparaging his other goofy ideas and lack of presidential temperament, Christie made clear that Trump was unfit to be commander in chief. Christie posed as the serious grown-up in this race on national security. That was the essence of his case to the American people.

In fact, after months and months of careful coaching by outside foreign policy experts, his initial gaffes (e.g. “occupied territories” was how he referred to the West Bank) stopped and he became proficient on national security.  One former adviser told me he “absolutely” would never have helped Christie had he known he would endorse Trump. He said of Christie’s endorsement, “It’s an absolute disgrace.” It is exactly that, because Christie knows better.

It is deeply sad he would then sweep away months of high-minded speeches to enable a man he knows to be unfit for the presidency to attain that office at a time of such urgency. I wrote recently that Trump’s rise was enabled by many people just “doing their jobs” — the mainstream media, talk show hosts, backbenchers in Congress, etc. But none will be as morally culpable as Christie if Trump succeeds. Why? Because he knows better, and because the threat — and it is a threat — of Trump as commander in chief, or anyone else unfit for that job, was a central motivation for Christie’s run.

No matter how cynical one becomes about politics, it seems never to be enough. One hopes that with this irresponsible, soulless act, Christie’s career comes to an end. He may well go down in history as the Republican most responsible for undercutting serious Republican presidential candidates and helping to elect Democrats whose policies endanger U.S. national security. Shame on him.