Trump Is No Longer Historically Unpopular
Everybody knows that President Trump started out with historically awful approval ratings. New presidents typically get a "honeymoon" period where the American people give them a chance to succeed. This approval bump may be diminished by circumstances. President Dubya came in after the traumatic election of 2000 with an approval rating that looked a lot like Trump's, and his honeymoon period saw it climb to just shy of 60%.
This argument was (CHOOSE ONE: only/already) 578 days ago.
Trump's "honeymoon" didn't even get him to majority support, not that it mattered anyway, because his "honeymoon" lasted about ten days before the controversy over the crowd size at his inauguration, the original travel ban order and its disastrously incompetent rollout, and the other daily controversies to which we have become inured chewed up all the good will the opposition was willing to give him.
This left Trump in an approval-ratings hole that was unprecedented for a president so early in his term, and, for a while, it seemed like it would just keep on going down, down, down until nobody could defend him and impeachment became a practical possibility.
This is no longer the case. Trump remains unpopular, but he is now pretty much within historical norms for unpopular first-term presidents.
Today is the 578th day of the Trump presidency. Here are the averaged approval ratings of all past presidents with data, as of Day 578 of their terms, sorted by highest to lowest approval.
I'm also including each president's approval rating on Day 1383, which is the week the president faces re-election, and whether he won or lost:
George H.W. Bush: 75.6% | lost re-election with 32.6% approval
LBJ: 69.3% | lost re-election; 38.7%
Eisenhower: 67.6% | won re-election; 67.9%
George W. Bush: 66.6% | won re-election; 48.8%
Kennedy: 66.1% | died on Day 1036 with 58.2% approval
Nixon: 55.8% | won re-election; 61.3%
Ford: 46.5% | lost re-election on Day 816 after partial term with 43.6% approval
Obama: 45.2% | won re-election; 49.2%
Clinton: 43% | won re-election; 56.7%
Trump: 42.0% | we'll see in 2020!
Reagan: 40.9% | won re-election; 57.9%
Carter: 39.9% | lost re-election; 37.9%
Truman: 33% | won re-election on Day 1300 with reported 39.6% approval, but polling was so new and spotty then that his approval rating hadn't actually been polled since June of election year. It seems obvious that Truman's approval rating went up quite a bit during the race to get him re-elected, though how much is anyone's guess
So President Trump is no longer the Most Unpopular President Ever. He is now the fourth-least-popular president since World War II. Nothing about his current approval rating is unprecedented. It would be entirely unsurprising if he went on to win re-election, just like Obama, Clinton, Reagan, and Truman... all of whom had approval ratings very close to or worse than Trump's at this point in their presidencies.
On the other hand, Trump's approval rating has been remarkably stable throughout his presidency. He's never gone below 37% or above 47%. The majority of his time has been spent in the low 40s, which is just where we find him today. He hasn't collapsed into impeachment territory (i.e. the mid-20s), as many thought he would, but he also hasn't been able to climb out of the hole he's in. If he can't change that by Election Day 2020, his odds of re-election do not look good. Most presidents who win re-election enjoy majority approval. (No shock there.) The only exceptions are George W. Bush and Barack Obama, who both had 49% approval ratings during the week of the election... which was close enough to a majority to scrape out decent victories in both cases.
Perhaps this era of negative polarization has changed the rules, and a President no longer needs majority support; a narrow majority of voters just need to see the President as less bad than the alternative. Indeed, that's how Trump won the 2016 election in the first place: Trump's favorability rating was horrific, but so was Clinton's, and enough Republicans held their nose for Trump to stop Clinton that he pulled out the win. It's worth noting that both of the re-election bids that succeeded despite sub-majority approval (Bush II and Obama) were also the most recent re-election bids, suggesting that perhaps negative polarization and "hold your nose" voting played a role in carrying them to victory as well. It's certainly not impossible for Trump to win a second term while carrying a 43% approval rating, especially if disapproval is largely contained to coastal states that are already voting against him while Trump holds on to slightly better numbers in critical Midwestern battlegrounds.
Still, it also doesn't seem all that likely. When faced with a novel theory of elections and mountains of precedent, the smart bet is to go with the precedent... yes, even in the age of Trump. President Trump probably needs to improve his ratings to win in 2020.
But, now that Trump's reached historically normal levels of unpopularity, there's a lot of precedent for that, too.