The most recent three midterm elections — 2006, 2010, and 2014 — have all been “wave” elections in which there was a backlash against the president’s party. Showing the popularity of the president plays a big part in how people vote.
President Trump currently has an average approval rating across all the polls of 37.6% and a disapproval of 57.6%. At this point in his first term December 2009, former President Obama had a 50% approval rating and a 43% disapproval rating.
The highest approval rating President Trump has had in any of the polls so far, even from those like Rasmussen and Fox, whose polls are consistently higher than most for Republicans is 46%, while Obama's highest rating was 69%.
The lowest Obama's ever fell to during the whole of his presidency was 38%, still higher than President Trump's current approval rating.
Now while anything is possible, after all there is a year until the midterms, things currently suggest that Republicans are going to struggle to keep the seats they already have, let alone win new ones.