Republicans were already at a massive disadvantage when it came to the 2016 Senate map — defending more than double the number of seats of the Democrats had. To compound matters, many of those endangered Republicans were sitting in swing state territory in a presidential year where the electorate already leans more liberal.
Donald Trump's once-sagging poll numbers rebounded nationally after cratering post-convention. He's doing better now in battlegrounds where he needs to win the White House — and where Republicans are defending their toughest Senate seats — but overall still narrowly lags Hillary Clinton.
Some Republican seats once thought to be sure-wins for Democrats, such as Ohio and Florida, are moving off the table. But now, seats in typically safe GOP turf, such as Indiana and Missouri, are at real risk of flipping. It's a much different path to the majority than either party had expected.