It was also a midterm election year. Under President Eisenhower, Republicans almost always lost ground in Congress during midterms. Plus the deep state needed some help to ensure 1958 was no exception. The only exceptions have come recently as 1998 (Democrats) and 2002 (Republicans) gained in Congress while holding the White House. I see 2018 as a wash as Trump lost the house but gained a seat in the US Senate.