Since he’s not going to get re-elected, he’s likely to vote with the democrats against Trump. Nothing to lose, nothing to gain by favoring Trump.
He actually does have a small chance to get re-elected, because there could be another Roy Moore type situation. For example, if he runs against Sessions, it should be a slam dunk for Sessions. But Sessions could be taken down in a scandal at just the wrong moment, bringing him neck-and-neck with Doug Jones in the race. Under that type of scenario, Doug Jones would have a better chance for re-election if he sided with Trump on the impeachment issue.