So the GOP front-runner’s campaign manager got arrested Tuesday for manhandling a female reporter based on unmistakable evidence from overhead video footage supplied to the police by Donald Trump’s own security folks — footage Trump and his campaign had told the world didn’t exist.

Now, you could look at this and shrug and say, well, it’s just another day that ends with Y when it comes to Trump. So maybe tomorrow some other aide will mug a nun. Maybe on Friday another will curb-stomp a disabled veteran. After which comes next Tuesday’s Wisconsin primary, and who is now willing to bet cash dollars Trump will lose that one?

Maybe Trump won’t, but this was definitely not just a day ending in Y for the thousands of Republicans who may find themselves appearing on a ballot in November beneath Trump’s name, from the House to the Senate to state legislative seats to mayor’s offices to school boards.

For them, it was a day for utter, bone-chilling, sickness-inducing panic, featuring screams akin to those heard by the unfortunates on the Titanic: “Where’s my life jacket?” “Every man for himself!” Only their screams are soundless, like Edvard Munch’s famous etching, because they have seven long months before they might drown, career-wise, on Election Day.

So Trump is standing behind his campaign manager, which fits with his nine months of talking about Megyn Kelly’s menstrual cycle and Carly Fiorina’s supposed ugliness and Heidi Cruz’s who-knows-what-he’s-alluding-to.

From one perspective, Trump’s loyalty is endearing. From another, it’s politically insane.

Look, there are certain facts about American politics that conservatives (like me) don’t enjoy hearing, but which are nonetheless true. Here’s one: Hispanic voters, who are overwhelmingly Democratic, are growing in number. Here’s another: Single women, who vote Democratic, are growing in number.

Thus, while Trump’s kindness toward Corey Lewandowski may strike some as nice, what’s not so nice for other Republicans is the fact that women made up 54 percent of the voters in 2012. What’s not so nice for Republicans is that Barack Obama won women by 12 percentage points in 2012.

What’s not so nice for Republicans is the $50 million ad buy Hillary Clinton’s campaign will make with Trump denying his campaign manager did anything and saying maybe he should get the reporter in question arrested for shouting questions at him.

What’s not so nice for Republicans is that Donald Trump will likely turn the gender gap into an abyss deeper than the Marianas Trench.

It’s also not so nice that, from a simple win-loss perspective, the GOP front-runner for president has an unfavorability rating among Hispanics nearing 80 percent. I wonder why that is . . . no, I don’t, really.

From one perspective, Trump’s loyalty is endearing. From another, it’s politically insane.

Mitt Romney got 27 percent of the Hispanic vote when he lost. Anyone want to hazard a guess just how low Trump could go? Ten? Five? There are 14 million Hispanic voters. Trump will lose the GOP another 3 million of them at least. And remember: The party already lost by 5 million in 2012.

The basic fact of presidential elections in the United States for the past generation is this: All things being equal, there are more liberal or Democratic voters during presidential years than there are conservative or Republican voters.

This was true in 1996, when Bill Clinton got 10 million more votes than Bob Dole. This was true in 2000, when Al Gore and Ralph Nader combined for 3 million more votes than George W. Bush. It was true in 2008, when Barack Obama got 9.5 million more votes than John McCain. It was true in 2012, when Obama got 5 million more votes than Mitt Romney.

This means, simply, that for a Republican to win the presidency in 2016 or ever again, the number of GOP voters has to increase substantially (and the number of Democratic voters has to decline a bit). In order to grow, Republicans have to be more attractive to voters as a general rule.

Every Republican running for office in the country who is not in a rock-ribbed Republican state or district knows this to be true. And yet Trump is presiding over a campaign supported by white males — the one part of the electorate Republicans have won by landslide margins throughout this period.

Trump has claimed the increased turnout of white males in the GOP primaries suggests he can create a stampede of them on Election Day. But even if he somehow does draw 5 million new white male voters to the polls who have never bothered to vote before, he would likely do so while turning a greater number of female and Hispanic voters into Democrats, at least for 2016.

Every day that ends in Y, Trump is shrinking the Republican vote.