I think there is a lot for Republicans to worry about in the House midterm elections; the results of special elections over the past year keep sending a red-alert warning.
Just as Republicans benefited from Obama's unpopularity in 2010 and 2014, the Democrats are benefiting from enthusiastic opposition to Trump. Looks to me like the Democrats are riding an "enthusiasm gap" that could sweep Nancy Pelosi to the Speakership.
Republicans just don't seem to feel the need to turn out in these elections. If the economy is supposed to be the magic bullet that will help Republicans, it didn't seem to help Rick Saccone in a Pennsylvania district that has been doing well economically and should probably have liked Trump's moves toward a tariff on steel. But despite his exuberant campaign appearance for Saccone, Trump couldn't pull him across the finish line.
While whistling past the graveyard, Republicans can tell themselves that it helped that Conor Lamb ran as a centrist.It's not a great sign when a party's best hope is that the other party screws things up. And all those candidates who benefited from Trump's win in 2016 are probably wishing he'd just get off Twitter and shut his mouth for the next eight months.