When democrats win the house, they can investigate essentially whatever they want. This includes subpoena power. So if trump thinks that things will get easier after the midterms, he’s gravely mistaken.
Furthermore, republicans used to have a very safe favorable map to hold the senate. Not so much anymore. Beto is closing in on ted Cruz. You should watch his 4 minute speech on NFL anthem protests. Gave me goosebumps.
By the end of the year we could be looking at democratic senators in both Alabama and Texas, two traditional republican strongholds. Trump isn’t exactly growing the party