In the first major, high-quality poll since last week’s debate, NBC News and the Wall Street Journal show Joe Biden building a 14-point lead over President Trump. As The Post’s Philip Bump reports, it’s merely the latest indication that Biden won the debate and may have increased his lead.

Almost immediately, Trump allies pointed to polls in early October 2016 that showed Hillary Clinton leading by double digits — including by the same 14-point margin in an NBC-WSJ poll. And she lost!

It’s always important to look at polling averages rather than just one poll. But the NBC-WSJ poll combines with other polls to show one major way in which Biden’s lead is different from Clinton’s, and they reflect perhaps the biggest failing of the Trump campaign — its inability to drag Biden’s image down.

In July, the same poll showed Biden’s image rating 12 points underwater, with 34 percent viewing him positively and 46 percent negatively. Since then, though, it has consistently trended positive: minus six in August (39 percent to 45 percent), minus two in September (43 to 45) and plus two (43 to 41) in the most recent poll. The new poll is the first time in any NBC-WSJ poll that Biden’s image has been in positive territory.

By comparison, Clinton’s numbers were regularly far underwater — by double digits for much of the 2016 general election. While that narrowed somewhat in the final two months, by Election Day, exit polls showed about 6 in 10 voters viewed her negatively, compared with 4 in 10 who viewed her positively. That was almost exactly the same as Trump, and Trump eked out a win thanks to some very close races in key Midwest/Rust Belt states.

Given that Trump’s approval rating has been mired in the low 40s for almost the entirety of his presidency, with a majority disapproving, there was always a premium on him pulling Biden down a la Clinton — or at least pulling Biden down enough to somewhere near parity. But the NBC-WSJ poll isn’t the only one to suggest that he has failed to do that at this late juncture, nor is it the only one to suggest that Biden’s image might actually be improving.

A poll from Monmouth University on Sept. 27, before the debate, also showed views of Biden about evenly split, if not nominally positive. While 47 percent had a favorable view of him, 46 percent had an unfavorable one. But between April and August, each of five Monmouth polls had shown Biden slightly underwater, by between two and eight points.

Other polls suggest Biden being more static in terms of his image but clearly not trending toward Trumpian or Clintonian levels. A Quinnipiac University poll ending Sept. 21 had his favorable-unfavorable split at 45 to 44, about the same as it has been since the Democratic primaries. And a weekly Economist-YouGov poll released Sept. 30 showed it at 50 to 48, the best since four weeks earlier.

The combined picture of all of them is that Biden is about equally liked and disliked, if not in slightly positive territory. And that’s unsustainable for the Trump campaign. As we’ve written before, even if the candidates were equally disliked, polls this time have regularly shown the “double-haters” — i.e., voters who dislike both candidates — breaking strongly for Biden, even as they broke strongly for Trump in the key states in 2016. So even if there were parity, it might not be enough for Trump. But there is not.

It’s a fact that also makes the timing of Trump’s coronavirus diagnosis all the more unhelpful for him. Trump has long eclipsed any news about Biden thanks to his domination of the headlines and constant controversy-stoking. Now an outbreak in the White House is dominating the news for the foreseeable future. Biden’s team has been content to run a quiet campaign and let Trump make himself the issue, and the Trump campaign and the candidate have done little to change that.

It’s a strategy that seemed to work for them in 2016, when Trump was constantly using his platform and campaign rallies that ran unfiltered on cable news to eviscerate his opponent. But today, the efforts to drag down his opponent clearly haven’t penetrated in quite the same way — either because Trump can’t get out of his own way or because Biden just isn’t as easily caricatured as an opponent. While Trump has repeatedly suggested that Biden would be a Trojan horse for the more extreme elements of his party, these polls suggest people don’t harbor the same reservations about Biden as they do about Trump or did about Clinton.

Either way, though, the practical implications are the same: Arguably the linchpin of Trump’s 2016 win — the approximately equal unpopularity of his opponent — isn’t at all reflected in the state of the race today. And that makes waging a comeback infinitely more difficult, even when Trump can move past his illness.

There were plenty of undecided voters and the polls were close enough in 2016 to allow a surprise. There’s not the same kind of margin for error in Trump’s direction in the 2020 polls right now.