Clinton pulled back on a kill shot for a reason: Keep Trump on the ticket

The second presidential debate in 60 seconds
The second presidential debate in 60 seconds

Donald Trump survived Sunday night's dark and bitter presidential debate but that's about all he did.

The bar has now been set so low for the GOP nominee that he could do the following on stage in St. Louis and still receive passing grades: suggest that as president he would jail his opponent; defend Vladimir Putin and Russia over the hacking of the U.S. election; praise brutal Syrian tyrant Bashar Assad; admit to not paying federal income tax; and rebuke his own running mate for daring to criticize Russia over the indiscriminate bombing of Syrian civilians.

That's a very partial list.

Trump also lied with enthusiastic regularity, again saying he opposed the second Iraq war before it started (he didn't), claimed his opponent would jack up the tax rate on the middle class (she says she won't) and protested that he did not Tweet at 3 a.m. that people should check out a sex tape featuring a former Miss Universe (he did).

And never mind that Trump spent the first portion of the debate apologizing for a video leaked over the weekend in which he grotesquely bragged about his ability to commit sexual assault because he's a "star." Diving deeply into the gutter, Trump tried to turn the video into a bizarre bank shot attack on Clinton by bringing to the debate women who claim former President Bill Clinton sexually abused them.

Trump, who himself faced a rape allegation from ex-wife Ivana Trump and now faces one from a woman who was 13-years old at the time the alleged act occurred (Trump has repeatedly denied the allegations) argues that Hillary Clinton is worse than he is because she attempted to smear and intimidate Bill Clinton's accusers, though the evidence for that smearing and intimidation is thin to nonexistent.

Trump also spent the debate wandering around the stage and regularly crowding Clinton's space and glowering over her shoulder like a stalker. When asked by a Muslim-American in the audience how he would deal with "Islamophobia," Trump repeated the debunked claim that witnesses saw "bombs all over the apartment" of the San Bernardino terrorists and failed to report it. There is no evidence that anything like this ever took place.

In any normal election season, Trump's debate performance would be graded a campaign-killing disaster. But 2016 is no ordinary year, and Trump was already so gravely wounded, with Republicans across the country rescinding their endorsements, that his ability to avoid a complete meltdown Sunday night ranks as a "win."

And Clinton for her part opted not to go for the kill shot. She mostly laughed off Trump's attacks and declined to bring up the sexual assault allegations against him, instead pivoting to her own vision for the nation's future. The lack of aggression irritated some Democrats who wanted to see Clinton knock down a staggering opponent.

But the high-road approach appeared to work with voters who declared Clinton the winner of the debate in multiple polls. And Clinton may have held back for strategic reasons, preferring to keep Trump alive rather than bait him into campaign killing moments that could drive him off the ticket in favor of Mike Pence. Clinton would likely win in any scenario but a Trump withdrawal would create significant uncertainty in a race that is now tilting back heavily in Clinton's direction.

Even before the explosive video surfaced over the weekend, Clinton had moved back to a nearly 5-point lead in the RealClear Politics average of general election polls. She has taken back the lead in the battlegrounds of Ohio and Florida and now appears to be a lock in Pennsylvania and Virginia. She has many paths to 270 electoral votes, Trump at the moment has none.

All the usual caveats apply. There is a final debate in nine days in Las Vegas. More damaging information could come out about Clinton. External events could shift the landscape in the GOP nominee's favor. But at the moment, the American people appear to have decided that while they may not like Clinton very much, they find her qualified to serve as president while Trump is not.

And Trump did just well enough in the second debate to stagger to the finish line of a race he is almost certain to lose.

CORRECTION: Donald Trump faced a rape allegation from Ivana Trump, not Marla Maples.

—Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.