His explanation:

With 38 GOP House seats rated toss-up or worse by Cook Political Report, to only three Dem Seats, 2018 Republicans have net 35 seats at risk, significantly more than they can afford to lose (23).

With 152% of Repubicans’ margin vulnerable, they’re in a more perilous position than Democrats in 2010 (when 123% of their margin was vulnerable) or Republicans in 2006 (119%).