If Biden wins by eight or more points nationally, Democrats should carry both of those Republican-held Senate seats. Biden should very narrowly carry Georgia if he wins by seven overall, but Georgia requires senators to win at least 50 percent of the vote to win the seat in November. If no one does that, a runoff is held in January between the top two finishers. Georgia is holding two Senate elections on Tuesday, one of which is a special election with multiple candidates, and I think both races will go to a runoff. In the other race, if Biden wins nationally by nine points or more, Democratic challenger Jon Ossoff should capture his seat outright. If Biden wins by six or less, Trump is likely to carry Georgia and Republican incumbent David Perdue should prevail.
House races strongly favor the Democrats because of the overall national climate. House Democrats carried the national vote in 2018 by roughly eight points, and Biden should win almost all of the 40 seats they gained from Republicans in that election. The former vice president is also likely to win many of the House districts Republicans kept but that Trump won narrowly in 2016. There are a few heavily Republican or GOP-leaning seats that Republicans will take back, but the number will pale next to the total Democrats pick up. As with the national race, most of the seats Democrats will gain are in suburban areas with large numbers of college-educated voters who are abandoning Trump’s GOP in droves.