Donald Trump’s biggest selling point is his brilliance as a manager.  

Yet if this week’s Republican National Convention is any guide, a Trump administration would marry the micromanaging of Jimmy Carter, who refused to delegate even the scheduling of the White House tennis courts, with the incompetence of, say, James Buchanan, who held that Southern secession was illegal, but that going to war to keep the country together was, too.  

The unlikely theme of the day here in Cleveland is “Making America One Again.”  

Doing that would never have been easy in a moment so polarized that even the weather is a political topic. (“It’s so cool here for this time of year,” my climate change skeptic dad often tells me in midsummer. “Really? Because it’s beastly here,’’ I often answer.)  

But this GOP convention has itself made “making America one again” a little harder.  

A conspiracy theory-loving person like the GOP nominee might not have to think too hard before concluding that yes, Donald J. Trump’s party is out to sink him with the goings-on here, which have featured daily booing on the floor by what he’s called a tiny minority of embarrassed losers.  

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John Kasich, the governor of the state hosting Trump’s nominating party, has not only stayed away but let slip that he was offered the vice presidency – and told by Donald Trump Jr. that if he took the job, his portfolio would include both foreign and domestic policy. (The nominee’s son has denied making any such offer, which would presumably have left Trump free to pursue other interests.)  

Then we have the whole Melania speech fiasco , George W. Bush wondering aloud whether he will go down in history as the last Republican president, and Ted Cruz urging Americans to “vote your conscience”  — a decision that the party, if it does survive, may yet come to see as principled even if it also involved score-settling.

After that performance, New Jersey Gov. Chris Christie called the Texas senator “selfish,” to which Cruz’s former campaign manager, Jeff Roe, responded, “That guy turned over his political testicles long ago.”  

All in all, you have to wonder how Trump can run a country when he can’t run a convention, or how he might bring the country together when the Quicken Loans Arena is the site of so many civil war  skirmishes.  

The speakers squarely in Trump’s corner are busy making the drawing together of our countrymen less likely with over-the-top rhetoric like Christie’s, when he essentially asked for a thumbs up or thumbs down during a mock trial of Clinton  on Tuesday that recalled entertainment in the Roman Colosseum . And when Ben Carson linked Clinton to Lucifer, Dana Carvey’s old “Church lady ” skit on Saturday Night Live was officially surpassed by reality.  

In his speech accepting the GOP nomination tonight, Donald Trump would have to change course radically to even start to unify the country. And though I’m sure we will hear a few words about love and kindness, any suggestion that we’re not that different or all that far apart would be so out of character that his supporters wouldn’t recognize him even if he did.  

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