There have been 13 presidents who have run for another term in the polling era (since 1940). For each of those presidents, I compared their average Gallup (or, in the case of 1944, the Office of Public Opinion Research) June approval rating and their estimated approval rating on Election Day.
The average president has seen his approval rating shift by just 3 points from now until the election. That would only get Trump into the mid 40s at best. Trump’s approval rating was similar during the 2018 midterms, when his party lost control of the House.
Net approval ratings tell the same story. The average president had his net approval rating shift by only 6 points from this point forward. Given Trump’s net approval rating is in the negative low to mid-teens, a 6-point improvement would land him with a net approval around -7 to -10 points on Election Day. Again, that’s about where he was during the 2018 midterms.