Duffy wrote that ever since a tape surfaced of Trump discussing making unwanted sexual advances toward women, Republicans have been slowly sinking in polls across the country.
Democrats needed four seats to win control of the chamber assuming Hillary Clinton wins the White House (thanks to the vice president's tie-breaker) or five without her.
"For much of the cycle, we have expected Democrats to score a net gain of between four and six seats. Since the release of the Access Hollywood tape, Senate Republicans have seen their fortunes dip, particularly in states like Florida, North Carolina, New Hampshire, Nevada and Pennsylvania where Clinton has established a lead," Duffy wrote.
While Cook still maintains there are seven toss-up races: Nevada, North Carolina, New Hampshire, Florida, Missouri, Indiana, Pennsylvania. Duffy explained that historically, one party typically wins the majority of those on the map.
"While the 2016 election has broken every political science rule and trend, we’d be surprised if this becomes one of them," she said.