Will the polls be right about Senate Democrats outperforming Harris?
In polling this cycle, Democratic Senate candidates have often overperformed their party's presidential candidate. For example, 538's final polling average in Arizona has Democratic Senate candidate Ruben Gallego up by 4 percentage points over Republican Kari Lake, while Trump is up by 2 percentage points in the state. Trump is also leading slightly in Nevada's state polling average by 0.3 percentage points, while the Democratic Sen. Jacky Rosen polled ahead of her challenger by 6. With smaller gaps, the same is true of each competitive Senate race in key swing states, with Democratic Senate candidates doing better than Harris in their respective races in Wisconsin, Pennsylvania and Michigan. At a time when elections are highly nationalized and split-ticket voting is down, can we expect these gaps to persist come Election Day?
To answer that question, I looked at October and November surveys leading up to Election Day in the 538 databases for competitive 2016 and 2020 Senate races (those where the final two-party margin was 10 points or less) that also included questions about that year's presidential race. I then averaged how each Republican and Democratic candidate polled and compared those averages with the election results.
For example, in Florida in 2016, Republican Senate candidate Marco Rubio led Democrat Patrick Murphy by an average of 5.4 percentage points in the polls, while presidential polling in the state gave the Democrats a smaller 1.9 percentage point advantage. That meant the polls had the GOP Senate candidate outrunning the GOP presidential candidate by 7.2 percentage points. Come Election Day, both Republicans prevailed, but Rubio outpaced Trump by 6.5 percentage points. So while the polls were off, they were just about accurate as to Rubio's overperformance relative to Trump.
By comparing those differences across states, we can estimate how well polls do at estimating over- or under-performance by Senate candidates. It wasn't just Rubio's race -- across the seven races we examined in 2016, the polls generally predicted which Senate candidates overperformed and which underperformed the top of the ticket, and the magnitude is typically in the right ballpark, too. The average difference in the polling gap and results gap between Republican candidates and Trump was an absolute difference of 0.9 points, with the biggest miss coming in New Hampshire, where Democratic Senate candidate Maggie Hassan and Hillary Clinton actually finished much closer than polls predicted.
The story is similar in 2020, as the chart below shows. Of the 10 states we analyzed, the polls predicted the direction of the overperformance in all but one. The exception was North Carolina, where the polls indicated that the Democrats' Senate candidate Cal Cunningham would narrowly outperform Biden. In fact, both candidates did a bit worse than polling indicated, though Cunningham was dogged by a late-breaking scandal and ended up underperforming Biden by under half a point.
Certainly, the polls didn't get the magnitude of the overperformance right in all cases in 2020. Take Maine, where the polls showed both incumbent Republican Sen. Susan Collins and Trump both losing, but Collins overperforming Trump by 7.8 percentage points. In fact, the gap was more than double that, with Collins going on to win easily while Trump lost, overperforming him by a whopping 17.7 points. Still, though the polls understated GOP support in both 2016 and 2020, they were much more accurate in identifying when a Senate candidate is likely to do better than the party's standard bearer.
This year, swing-state Democrats may be hoping to take a page from Collins' book, playing up their bipartisan, moderate credentials and previous work with Trump in an effort to win over split-ticket voters. Recent history suggests that their current overperformance in the polls is a good sign for them, though certainly not a sure thing.
