For months now, President Trump and his reelection campaign have been focused on communicating a concerted message to potential voters. Not Trump’s plans for a second term, which have been ... gauzy, even when Trump has been asked about them by friendly interviewers.

No, the focus has instead been on Trump’s likely opponent, former vice president Joe Biden. Biden, Trump would have you believe, is both unfit to serve and potentially dangerous to the republic. Voters don’t seem to be buying it.

It’s understandable why Trump would take this tack. After all, the unpopularity of his opponent was central to his 2016 victory. Both he and Hillary Clinton were historically unpopular in terms of their favorability — and Trump managed to parlay that into just enough votes to win the electoral college. So why not try it again?

Well, two reasons. The first is precisely that Trump’s efforts to dirty up Biden haven’t really gotten anywhere and — more importantly — it’s not clear that a race between two unpopular candidates actually favors Trump in the same way it did four years ago.

YouGov polls regularly on views of the candidates, as it did in 2016. You can see how each contest has evolved from the numbers the firm provided to The Post. Clinton and Trump were both seen as more unpopular than popular over the course of 2016, which each having a negative net favorability. (Net favorability is the percentage of poll respondents who view the candidate favorably minus those who view the person unfavorably. On the charts below, the candidate whose line is higher is viewed more favorably on net by the identified group.) So are Biden and Trump, with one difference: Biden is viewed more positively than Clinton was, and Trump is viewed more positively than he was four years ago.

Note the obvious bump for Trump earlier this year, when the coronavirus pandemic emerged. That faded. Trump’s current net favorability, in fact, is about where Clinton’s was in 2016 — which, for obvious reasons, isn’t really good news for his reelection bid.

It’s also not good news when considering why Trump’s net favorability is higher now. It’s almost entirely a function of how members of his own party view him. In 2016, coming off a contested primary, Trump had to build respect among Republicans. Eventually, most of them voted for him anyway, even if they didn’t love him. Now, he’s more popular than Clinton was among Democrats four years ago.

Biden, facing a contested primary of his own, is on the same trajectory Trump saw in 2016, gaining ground with Democrats. His net favorability with his own party has already matched Trump’s; if the same pattern holds, he will soon pass the president.

Again, this matters because of the importance of voters who view both candidates unfavorably. In 2016, about a fifth of voters fell into that category. They ended up preferring Trump by a 17-point margin — and by enough in Wisconsin, Michigan and Pennsylvania to hand Trump the presidency.

Why? Probably in part because he was a blank slate, someone whose lack of experience in politics allowed people to compare what he might to do with what they expected from Clinton based on what they knew about her. And so he carried those voters and won the election.

On Tuesday, Monmouth University released a new poll looking at the same group, those who view both major-party candidates unfavorably or, in the case, of Monmouth, viewed neither candidate favorably. Whom do they prefer in a head-to-head contest?

Biden, by a mile.

I don’t want to be hyperbolic, but if I’m Trump’s team and I see that chart, I start to panic.

There are lots of ways in which they might assure themselves that things aren’t that bad. After all, how many people who really dislike both candidates are going to bother to vote? What’s more, Trump’s stronger position with Republicans will likely mean that he can expect more votes from his own party (although the percentage of Monmouth respondents who view only Trump favorably is about equivalent to the percentage who said the same thing in 2016 exit polls). And, of course, there are still nearly three months in which to erode Biden’s support.

Which brings us back to the point that those efforts haven’t made much headway yet. In fact, this same metric hasn’t really budged since December. Then, Biden had a 33-point advantage among dislike-both respondents in a poll conducted by Quinnipiac University. In April, his advantage was 32 points.

Part of this certainly reflects that, even while both Biden and Trump are viewed negatively on net, Biden is viewed less negatively. YouGov’s data shows him faring better with Republicans than Trump is with Democrats, which obviously makes an important difference. Trump is about where he was with Democrats four years ago at this point; Biden is doing about 10 points better than Clinton was with Republicans.

Part of Trump’s victory can also be ascribed to the late swing of independents toward his candidacy. His net approval with independents in YouGov’s polling is currently about where it was right before the 2016 election, after that late surge. Biden’s net approval is about the same as Trump’s — but much higher than Clinton’s was.

There’s an important caveat here that should also calm nerves in MAGAland. Voters are much more familiar now with Biden than they were at the beginning of the year, when about 15 percent didn’t know enough to have an opinion. But it’s still the case that about 1 in 12 (largely independents) don’t know much about Biden, giving Trump and his team theoretical space to define their opponent.

We have to reiterate, though, that they have been trying to do this for months. Biden’s approval among independents and Republicans hasn’t really budged, and his approval with Democrats has surged.

There’s another potential problem with that lack of familiarity, too. If voters go into the polling place in November (or, really, fill out their ballots at home) and have to choose between the Trump they know and the Biden they still don’t know a whole lot about, many may go with the blank-ish slate that the Democratic candidate somehow represents.

If anyone should know the political benefit of being seen as an undefined outsider, it’s Trump.