Donald Trump speaks in Aiken, S.C., on Saturday. (Richard Shiro/Associated Press)

Rachel Maddow hosts MSNBC’s “The Rachel Maddow Show” and is a contributing columnist for The Post.

In the three years since the massacre at Sandy Hook Elementary in Newtown, Conn., one of the more inexplicable aspects of its aftermath has been the persistence of the insane conspiracy theory that the killings never really happened.

Sandy Hook “truthers” contend that the incident was either faked in its entirety, or, if it was real, was committed by the government for some sort of political effect.

I’m an optimist about the basic decency of the American people, so I prefer to think that the Newtown “false flag” myth is too repulsive and cruel to persist organically on its own merits. As far as I can tell, the main reason it’s still with us today is because it has been embraced by America’s industrial-size factory farm of conspiracy theories: the radio show and Web empire of Alex Jones.

Jones says, “Sandy Hook is a synthetic, completely fake, with actors, in my view manufactured. I couldn’t believe it at first. I knew they had actors there, clearly, but I thought they killed some real kids.”

On the morning of Dec. 2, Jones hosted an extended, live interview with Republican presidential front-runner Donald Trump. After about 30 minutes of mutual compliments, and Jones telling Trump that “about 90 percent” of his listeners support him, the presidential candidate wrapped things up by telling Jones, “Your reputation is amazing.”

That same day, after that interview, 14 people were killed and 21 others were injured in the mass shooting in San Bernardino, Calif. Within hours of that news breaking, Jones and his website — predictably — were hosting discussions of how San Bernardino, like Newtown, like the Boston Marathon bombing, and of course like 9/11, was a hoax. Either it didn’t happen, or if it did, it was perpetuated by the government to bring about . . . who knows, gun control, maybe? Mind control? Something about aliens? Concentration camps?

Mr. Trump is not the only presidential candidate to flirt with this particular strand of the American fringe. Sen. Rand Paul (R-Ky.) and his father before him have made frequent appearances in that wild territory for years. But aside from an outsize media fascination with each of them, neither Paul has ever been anything other than an asterisk in presidential politics.

Trump is a different punctuation mark entirely.

Now, the guy trading compliments with Jones is the Republican Party’s likely nominee — the candidate who is all but lapping the rest of the field in recent polling.

It’s one of those situations in which the whole is more than the sum of its parts: It may not be that crazy that Trump is embracing Jones, and it may not be that crazy that Trump is leading the Republican presidential field. But to have the leader of the Republican presidential field embracing someone like Jones? That’s nuts. That’s something new.

During a rally in New Hampshire on Sept. 17, Republican presidential candidate Donald Trump did not challenge an audience member who said that President Obama is Muslim and "not even an American." (AP)

In this new political reality, it’s time to figure out what it will mean to follow the leader of the Republican Party.

Acceptable affiliations and positions in that party are now effectively being set by a candidate who still maintains that President Obama is secretly foreign, who apparently seriously intends to hang a “No Muslims Allowed” sign on all U.S. borders and who finds “amazing” America’s foremost trafficker in the almost-too-disgusting-to-repeat lie that the grieving families of all those little 5- and 6-year-old boys and girls in Newtown faked their children’s deaths for some nefarious political cause. Amazing indeed.

Iowa and New Hampshire are just weeks away — the time has passed for either surprise or denial about the rise of Trump as the GOP’s likely 2016 nominee. It’s time to face the fact that Republican voters are far enough into this process that they know what they’re getting with Trump, and they’ve decided that they like it, more and more all the time.

What I’m genuinely perplexed by, though, is how it’s going to change the Republican Party — how it’s going to change what counts as normal and acceptable in that party — to have Trump as its standard-bearer.

When Popular Mechanics published its seminal debunking of 9/11 conspiracy theories, Sen. John McCain (R-Ariz.) wrote the foreword to that book. In his 2008 presidential campaign, McCain was happy to set straight his own supporters who called then-Sen. Barack Obama “an Arab” and more besides.

Mitt Romney was not exactly a profile in courage against the nuttier elements of the 2012-era GOP base — who can forget his bizarre Trump-endorses-Romney extravaganza in Las Vegas, complete with a gold-embossed, Trump-branded podium and a big bank of American flags?

But when pressed on Trump’s signature hangup about Obama’s birth certificate, Romney was straightforward and clear: “There’s no question about where he was born. He was born in the U.S.”

Leadership has its burdens. Among them is the necessity of correcting and redirecting the well-meaning folks on your own side who are attracted to dangerous conspiracy theories.

There will always be someone out there cooking this stuff up and selling it to the unhinged and gullible. But that just makes it a workaday responsibility of political leaders to make sure this stuff doesn’t get mainstreamed. I’m not saying we’ve always been great at that, but at the level of presidential nominees, we’ve at least recently had the benefit of candidates who see it as their responsibility to try.

Who will take on that role in the 2016-era Republican Party if and when it decides that the wild-eyed conspiratorial fringe isn’t just welcome on the edges of their party anymore ; it’s in charge? Who can?