The man who intended to murder Donald Trump on Sept. 18 has explained in his own words why he wanted to do it.

The words aren’t only his, though — they’re also the slogans and cliches of Trump’s Democratic opponents, virtually verbatim.

Ryan Routh left a letter with a friend, months ago, to apologize in case his assassination attempt failed.

What’s been released so far makes Routh’s political motives clear — he saw Trump as dangerously unfit for the presidency and a threat to peace.

Routh was especially incensed Trump terminated Barack Obama’s deal with Iran:

Trump, he wrote, “Ended relations with Iran like a child and now the Middle East has unraveled.”

America’s president, according to the would-be killer, “must at bare minimum embody the moral fabric that is America and be kind, caring, and selfless and always stand for humanity.”

If this all sounds familiar, it’s because Joe Biden, Kamala Harris and Trump’s legion of Democratic and ex-Republican detractors have supplied the premises for stopping the former president by any means possible.

After all, if democracy really is at stake in November, voters themselves must be suicidal even to consider voting for Trump.

To save democracy in such an emergency might well seem to call for extreme measures of a sort that no politician would dare endorse — but that a true believer might be prepared to hazard.

Harris, Biden, and other Democratic leaders are quick to lament political violence; they have a responsibility to do more, however, by acknowledging that whatever their differences with Trump, his election does not mean the end of constitutional government.

President Trump was not a tyrant, he isn’t poised to become one if re-elected — and, most importantly, killing him is nothing like justifiable tyrannicide.

If Democrats could bring themselves to say this, they would do wonders for the emotional health of some of their own supporters — and might just save lives, including but not only Trump’s.

The chilling thing about Routh’s letter is that it’s not a mindless, stream-of-consciousness rant: it’s an assassin’s manifesto written in the bland language of ordinary anti-Trump discourse.

Like Routh in his letter, in the 2020 presidential campaign Joe Biden and New York Democratic Sen. Kirsten Gillibrand both described Trump as a menace to America’s “moral fabric.”

“One of the worst things about President Trump . . . is he’s torn apart the moral fabric of who we are,” Gillibrand said in one Democratic debate, while Biden accused Trump of shredding “America’s invisible moral fabric.”

Routh’s “kind, caring, and selfless” description of a good president — one he presumably wouldn’t murder — sounds generic, yet closely tracks Kamala Harris’s endorsement of Biden four years ago as “kind and endlessly caring” and possessed of “selfless courage.”

Those words are obviously not incitements to violence: they simply testify to how little original thinking was taking place in Routh’s head.

In the empty space between his ears there was a great echo chamber, reverberating with whatever he heard from Democrats and the anti-Trump hype machine.

Routh wasn’t just soaking up the syllables of anti-Trump rhetoric, he was internalizing a mindset that presents Trump as an enemy of humanity itself, not only immature but malevolent.

Democrats have created an extraordinary climate of fear around Trump, and if they don’t weigh their words more carefully, more violence will follow from the likes of Ryan Routh.