He ought to realize that he's not just hurting Trump here; he is actually is hurting Republicans across the entire nation because of down-ballot voting. For the sake of the party, he needs to play his role.
How does helping (or not) Trump perform well in Ohio have any bearing on downballot voting in other states?
A) Because you have a lot of uninformed voters who only go to the polls to vote for President, and then lazily start selecting everyone in that same party down the line.
B) Because our nation doesn't have a media blackout on releasing poll results until the polls in the last State have closed.