Former Vice President Joe Biden leads President Donald Trump in pretty much every single national poll. Yet the same polls find that Trump's supporters are much more enthusiastic about voting for their candidate than Biden's supporters are voting for theirs.
This split is potentially a good sign for Trump because the candidate who has led on enthusiasm (or a closely related question) has won every presidential election since 1988, though there are reasons to think Biden could break this streak.
Importantly for Trump, the leader on enthusiasm has gone on to win in close elections as well as ones with wider margins.
One of those close elections was four years ago. Trump had a consistent edge over Hillary Clinton in enthusiasm. His voters were 4 points more likely to say they were very enthusiastic in voting for him than Clinton's were for her in the final ABC News/Washington Post poll, even as Clinton led overall. That enthusiasm advantage should have been one of the warning signals to the Clinton campaign.
Trump's current edge in enthusiasm over Biden is even larger. In a late March ABC News/Washington Post poll, 53% of Trump backers said they were very enthusiastic about voting for him. Just 24% of Biden backers said the same about their guy.
This 29-point difference would be the largest on record, if it held through the election.
The largest prior gap at the end of the election that I could find was in 2008. Back then, 67% of Barack Obama voters said they were very enthusiastic about voting for him. A mere 41% of John McCain said they were very enthusiastic about voting for their candidate. McCain, like Biden, was someone who had run for president previously and was seen as closer to the center than most of his primary rivals.
Before Biden, the most recent candidate who had run for president twice before winning his party's nomination was Bob Dole. Dole lost. A minority (40%) of Dole voters said they strongly favored him in an October 1996 CBS News/New York Times survey. The majority said they did so with reservations or because they didn't like his rivals. Bill Clinton's voters were 10 points more likely than Dole's to say they strongly supported him.
If nothing else, enthusiasm can manifest itself through the difference between registered and likely voters. If one side is more enthusiastic about their candidate, you could see them benefit by doing better among those who actually cast a ballot as compared to all voters.
Yet, there are a few reasons to think that 2020 will end up differently than these past years in terms of enthusiasm.
Actually asking people who they're going to vote for is much more predictive than looking at enthusiasm. Since 1988, the average difference between the margin in the final national polls of likely voters and popular vote has been 2 points. During that same period, the average difference between the enthusiasm edge margin and the popular vote was 7 points. That is, you'd rather be ahead in the topline number than enthusiasm.
(Remember, even in 2000 and 2016, the candidate who won on enthusiasm lost the popular vote.)
Moreover, there seems to be little correlation between how much a candidate outperforms their polling and enthusiasm. In other words, a candidate ahead on enthusiasm may do better than their polling, but it's far from a guarantee. Obama, who had the largest enthusiasm edge on record in 2008, did about as well as the final polls showed.
The last thing I'll note is that there is a history of one side winning even if their voters feel less enthusiastic -- and it's been in midterm elections, per ABC News/Washington Post polling.
In October 2006, Republicans voters were 2 points more likely among those certain to vote to say that they were very enthusiastic about voting for their candidate. Republicans got trounced anyway and lost the House.
In September 2014, Democratic voters were 10 points more likely among those certain to vote to say they were very enthusiastic about voting for their candidate. Still, Republicans held an overall 3-point lead among those certain to vote, and this advantage held through the election.
Put another way, Biden's got a real shot at winning if voting patterns match that of a referendum election like they do in a midterm, even if he has less enthusiasm. As I've noted, 2020 does look like a referendum.
If Trump can turn 2020 into a choice, his enthusiasm edge over Biden may matter more.