Does Trump have momentum?

And what does “momentum” mean in the context of an election forecast?

Well, it happened.

It was almost inevitably going to happen at some point, given how close the forecast has been to 50/50 since Kamala Harris took over for Joe Biden. But for the fourth time this cycle, the streams have crossed, and the nominal favorite in the race has changed. Thanks in part to one of the first high-quality national polls in weeks to show him ahead — yes, it’s from Fox News, but Fox News polling has no history of GOP bias — Donald Trump now has a 50.2 percent chance of winning the Electoral College.

That’s not much different from Trump’s 49.4 percent chances yesterday: the difference is that he wins the Electoral College an additional 1 time out of 125. But Trump is past the 50-yard line — and above Harris’s chances, which are 49.5 percent¹ — for the first time since Sept. 19. (Harris remains a clear but not overwhelming favorite in the popular vote, but that’s not how American elections are decided.)

To be open about my editorial thought process, it can be hard to know how much to emphasize these small shifts. Outside of major events like debates, the model typically does not move very much from day to day. And there’s no special significance to the 50-yard line. A change from 49.4 percent to 50.2 percent does not particularly matter more than one from 43.1 percent to 43.9 percent or 52.5 percent to 53.3 percent. 

I’m also aware that most coverage on the polling beat tends to overemphasize these small changes. People treat a 51/49 forecast very differently from a 49/51 forecast when they shouldn’t. There are also some partisan asymmetries here: Republicans tend to gloat about polling leads, while Democrats tend to live in fear they’ll evaporate. And Republicans more actively cultivate narratives that their candidate has momentum in the race.

But life is full of arbitrary numerical thresholds: hitting 50 home runs is a much bigger deal than hitting 49, and your 40th birthday is a much bigger deal than your 39.

And it’s not like these small changes have offset one another: lately, the daily shifts have mostly worked in Trump’s favor. He bottomed out at a 41.7 percent win probability on Sept. 27, about two weeks after his debate with Kamala Harris. Since then, his odds have improved day-over-day 12 times, decreased 7 times, and been unchanged once. So that’s not an overwhelmingly consistent pattern, but it starts to add up. I want to avoid the sort of boiling frog syndrome where you ignore a trend because it happens slowly and incrementally.

Plus, for what it’s worth, the Silver Bulletin model has been slightly more explicit about showing a trend toward Trump over the past week or so than other models — matching prediction markets in that respect. Although, prediction markets are probably going too far in this case and can overstate the importance of momentum.² 

I’ve already used the “m-word” twice — momentum — without formally defining it, and the formal definition is important. Does the fact that Trump’s chances have increased from 42 percent to 50 percent imply that they’ll continue to increase? Hold that thought, though, because I think I owe you a rundown of where Trump’s odds have improved. There are some pretty consistent patterns at the state level: