Historically, midterm elections are difficult for the president’s party. More specifically, the president’s party has lost House seats in 19 of the last 21 midterm elections, including an average loss of 33 House seats in those 19 cycles. Republicans lost 40 seats in Trump’s first midterm election.
Usually that happens when voters are dissatisfied with the direction of the country and don’t have the opportunity to voice their disapproval against the president because he’s not on the ballot. So they take out their frustration on the president’s party.
Midterms are fundamentally a referendum on the president — unless the dynamic changes.
If Trump is campaigning, tweeting and generally just being himself, he’ll interject himself into the spotlight because ultimately that’s where he’s most comfortable and because the news media has been unable to ignore him since 2012. That might excite the GOP base, but it could also remind independent voters why they voted against him and excite Democratic voters. Trump’s trail of tweets and actions could also command attention and force Republicans to answer for him, even though he’s not in office.