Usually, we wouldn't even think to ask whether a president with around a 40% approval rating would end up hurting his political party. It was among the 18% of voters who didn't like either candidate that Trump won the election. Still, there was no inherent reason to think that Trump would clean up among those who disliked both Clinton and Trump. We can simply look at Trump's approval rating because he's the incumbent and midterms are generally about feelings towards the President. Trump has maintained an approval rating of about 40% throughout most of his presidency.

Source: CNN May 05, 2018 13:51 UTC