First, Trump’s approval rating could change. Trump has almost a year left before the 2018 midterms—which is more than enough time to improve his approval substantially (or drive it further downwards).
Second, the House GOP’s floor may be higher (and its ceiling lower) than the president’s. In November of 2008, President Bush’s job approval was in the 20s, but Congressional Republicans managed to get 42.6 percent of the House popular vote. By the same token, in November 2002, Bush’s approval rating was in the mid-60s when Republicans held the House with 50 percent of the vote. In other words, there are some voters who will stick with their party even if their opinion of the president changes.
Third, Democrats haven’t yet convinced everyone who disapproves of Trump that they should vote for them.