The non-partisan Cook Political Report on Tuesday predicted that Democrats will gain five to seven Senate seats in the Nov. 8 election, an outcome that would strip Republicans of their majority.
Democrats need to make net gains of at least five seats to win a 51-vote majority in the 100-member chamber. If they gain four seats and Hillary Clinton becomes president, Democrats would have a de facto majority because the vice president would side with his party to break any ties.
The Cook report, a leading political handicapper in Washington, said Republican senators and candidates have Donald Trump to blame for their plight. The report cited the release of the infamous Access Hollywood tape of Trump making lewd comments about grabbing women by their genitals.
"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," wrote Jennifer Duffy, who analyzes Senate races for the Cook report. "In fact, of the Senate seats in the Toss Up column, Trump only leads in Indiana and Missouri where both Republicans are running a few points behind him."
Before the tape's release, Cook analysts had expected Democrats to gain four to six seats, Duffy wrote.
"We feel that the prospect that Democrats will have at least 51 seats is greater than the odds of a tied Senate, or of Republicans somehow holding their majority," she wrote.
The Cook report's analysis differs slightly from that of the non-partisan Rothenberg & Gonzales Political Report, which lists the Florida race for Senate as "leaning Republican" rather than as a toss-up. Republican Sen. Marco Rubio is running for re-election against Democratic Rep. Patrick Murphy.