The unions are Democratic Party strongholds. Trump complicated that for them, because the actual workers in most unions were supporting Trump. So the union leadership has had to toe a line between the politicians they have traditionally had their corrupt relationships with, and the rank and file they need to keep happy in order to keep their power in the union.
It's about $$. Richard Trumka had a net worth in the millions of dollars when he died a few years ago, He didn't make that money in the mines. He made it mining the union. The leaders of the unions need the support of the people in the unions as much as they need political preference from politicians. If they lose the rank and file, they lose their gravy train. And if the rank and file are die hard Trump, the way they are, I don't think they will ever "mobilize" against Trump. They'll do what they have always done with Trump, and try to play both sides of the fence in a way that doesn't alienate their members.