This isn’t complicated.

Missouri state Sen. Maria Chappelle-Nadal has to go. The Democrat from University City must do the right thing and resign her office immediately for her incendiary Facebook post expressing hope that President Donald Trump would be assassinated.

That is crossing a big, bright line, especially for a public official as experienced as Chappelle-Nadal. The veteran lawmaker, 42, knows better. It matters not that she promptly deleted the post, knowing full well that she had done wrong.

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And it matters not that on Sunday she expressed her regret directly to the Trumps.

“I made a mistake,” she said. “President Trump, I apologize to you and your family.”

Upon issuing that apology, Chappelle-Nadal indicated no willingness to step down. “I will continue to fight for issues that are really important,” she said.

But Chappelle-Nadal understands that there are certain no-nos in public life that routinely result in forced departure from office: Appropriating tax dollars for personal use. Using your position to procure personal favors. Sexually harassing interns. Expressing completely ridiculous and harmful thoughts such as the concept of “legitimate rape.”

An elected official writing “I hope Trump is assassinated!” on a social media site clearly falls into that category, especially in the toxic political climate in which we find ourselves.

By the way, threatening a president is a federal crime. The Secret Service is investigating, and that by itself could lead to a forced resignation.

As we said, this isn’t complicated.

Missouri leaders from both major parties, including Gov. Eric Greitens and Sen. Claire McCaskill, have demanded that she resign. McCaskill, a fellow Democrat, went right to the point:

“I condemn it. It’s outrageous. And she should resign,” the senator said in a statement.

Public officials have a special obligation these days to demonstrate appropriate ways to deal with the wildly controversial issues of the moment, so many of which have taken on life-or-death hues. Without a doubt, the president is guilty of ramping up the partisan intensity to almost unbearable levels at times. So many times his own word choices have proven instantly regrettable.

Still, blaming this on Trump is no excuse.

This issue won’t drift away. It’s too simple to grasp — and the sentiment too vile. The longer Chappelle-Nadal stays, the more damage she does to her party’s improving prospects for 2018.

Chappelle-Nadal and controversy have been closely linked for years. During the Ferguson uprising in 2014, she used a cardboard cutout of former Gov. Jay Nixon’s head as a prop to repeatedly express her disdain for the way the governor from her own party handled that crisis. On Twitter, she cursed Nixon, using an f-bomb in the process.

But this latest controversy stands apart.

It’s reprehensible. It’s an embarrassment to the entire state and a message that has no place in American discourse today.

Senator, you’ve got to go. At this point, it’s the least you can do.