Sure he could, though I'd say the odds are still against it. But this could be a surprisingly competitive contest in 2020. The latest evidence for this is a recent Quinnipiac poll of Texas registered voters, which ran several trial heats of Trump against named Democratic contenders. Three candidates, Biden, Sanders and O'Rourke almost tie Trump, losing by 1, 2 and 1 points, respectively. Harris and Warren lose by 7 points and Castro by 5 points.
The internals of these trial heats are interesting. Looking at Biden's demos--which are quite similar to Sanders' and O'Rourke's--he loses white college voters 54-37 and white noncollege voters by 70-27. These margins may look bad, but, according to States of Change data, are actually quite a bit better than Clinton's in 2016, particularly among white noncollege voters. Biden's Hispanic margin is basically the same in this poll as Clinton's in 2016 and, intriguingly, actually a bit better than Castro's in his trial heat.
Another positive sign for Democrats in this poll is that O'Rourke ties Cornyn in a Senate trail heat. Food for thought.