If history were to repeat itself in 2018, this would give Democrats the Senate and keep the House in Republican hands. History rhymes, it doesn’t repeat.
Jimmy Carter was very popular at this point in his term, in the wake of the Camp David accords. His approval rating was 56%. The fact that Republicans won in 1978 despite this shows their issues had saliency. The Wikipedia entry on the election notes that it set the stage for the Reagan Revolution, and that the Republicans who won were far more conservative than those they replaced.