Let’s get one caveat out of the way: We don’t have that many public polls testing Harris against Trump. From April 1 through July 2, just over a dozen polls asked about this alternative matchup. But we do have polls from all the major swing states, thanks largely to tracking from Morning Consult, and we have enough national surveys to calculate a Harris-versus-Trump national polling average — and thus to forecast how she would perform in states without any polls.
For the most part, national polls have shown Harris doing about the same as Biden in head-to-head polls against Trump. In a March Fox News poll for example, Trump led Harris by 6 points and Biden by 5 points (well within the survey’s margin of error). And as recently as June 28, a Data for Progress poll showed the president and vice president each losing to Trump by 3 points (also within the margin of error). That said, a June 28-30 CNN/SSRS poll found Harris losing to Trump by only 2 points while Biden was trailing by 6. This was also within the margin of error but was nonetheless a bigger gap and could mark the beginning of a shift for Harris.
When we plug all these polls into a polls-only version of the 538 forecasting model — which jettisons the economic and political priors our full model uses, giving us an apples-to-apples comparison between candidates — Harris has a slightly higher chance of winning the Electoral College than Biden, but it’s not a significant difference: 38-in-100 versus 35-in-100. On a state-by-state level, Biden looks stronger than Harris in Pennsylvania and Wisconsin, while Harris’s odds are higher than Biden’s in Nevada.
Harris also does slightly better than Biden in our forecast of the national popular vote. The model forecasts that Trump would outpace Harris nationally by 1.5 points, while he would outrun Biden by 2.1 points. However, this could be an artifact of our model not having any Harris-versus-Trump polls that include independent candidate Robert F. Kennedy Jr., who tends to take slightly more votes away from Democrats than Republicans when included in a poll.
However, Harris’s popular-vote edge is almost entirely negated by the bigger Electoral College bias against her. In our polls-only forecast pairing Biden against Trump, the Democratic candidate needs to win the popular vote by just 1.1 points to win the presidency. That’s thanks to Biden doing better in Pennsylvania, the likeliest tipping-point state in our model. Harris, by contrast, would need to win the popular vote by 3.5-4 points to win Pennsylvania and, with it, the Electoral College.
However, whether Harris would truly be a stronger candidate than Biden also depends on information besides the polls. In our full forecast model — which includes a variety of non-polling economic and political variables, which we call the “fundamentals” — Harris does much worse than Biden across the board. Whereas Biden has a 48-in-100 chance to win the Electoral College, Harris has only a 31-in-100 chance.
This is thanks in large part to the boost our model confers on Biden as the incumbent president, which is worth an extra point for Biden over Harris in our fundamentals-only forecast of the national popular vote. However, one factor our model does not consider is whether presidents’ approval rating and economic growth impact incumbents running for reelection more than non-incumbents running from the same party, and that may actually push Harris’s numbers over Biden’s. In other words, your mileage may vary depending on how much you believe that Biden should get a boost because he’s the sitting president. There is no objectively correct answer here; one of the reasons election forecasting is hard is that it requires judgment calls like these.