Even if he's a staunch anti-unionist, he'd still be considered "the most pro-union president in history", because the bar is located fifty floors below the basement of my deep sea research base at the bottom of the marianas trench, and all he has to do is not directly give the order to massacre a group of striking workers on his first day to pass it.
Not really even… I wouldn’t even call Truman the most pro union president or whatever (he might also be the worst president? it is so hard to choose), but he tried to seize the steel industry away from its owners to avoid a strike… because he saw the industry as responsible, not the union… he also vetoed the taft-hartley act and refused to use it to break the strike
The cool and logical thing to do would have been to nationalize the railways, but that probably never even occurred to a single democrat politician