The President's party just doesn't keep the House in a midterm election, especially since their approval rating isn't well over 50% but actually is really bad.
It's misplaced to claim this is an absolute rule. In the Gilded Age elections from 1876 to 1892 (closest equivalent to our current one in polarization levels and in persisently close elections) the results were as follows:
1878 midterm sees incumbent D House majority re-elected, R midterm
1882 midterm sees incumbent R house majority defeated, Ds take control, R midterm
1886 midterm sees incumbent D house majority re-elected, D midterm
1890 midterm sees incumbent R house majority defeated, Ds take control, R midterm
Out of these 4, the President's party holds the House once.
Expand it past two more elections and you have one R tidal wave (1894) and one election in which Rs keep the House (1898). Two out of six - one for each party.
History shows that it's evidently possible for the President's party to hold the House, even if we ignore elections such as 2002. We don't yet know how much luck there will be this time around though - it's really up in the air and how well Ds play their cards next year will likely be quite decisive.
