The first presidential election in which every state picked their electors via statewide popular vote was 1868. Before that, at least one state had the legislature pick the winner (South Carolina was the last holdout). Back then, only white men over 21 could vote, but the electorate saw significant boosts in 1920 with the passage of the 19th amendment (white women) and 1972 following the civil rights movement (black men and women) and the passage of the 26th amendment (18 year olds).
The total number of eligible voters grows with the overall population of the country, and looking at the data since 1972 we can predict that there will be about 244 million eligible voters this year.
But eligibility is only half the equation. Actual voter turnout never even comes close to 100%. Since 1972, it has never gone above 66.6% (in 2020), averaging 57.2%
57.2% of 244 million voters would mean we'd see 140 million votes cast in November, but that's actually way lower than we would expect. Let's ignore turnout percentage (which fluctuates wildly between 50% and 60%) and look instead at the actual number of votes cast:
There were over 158 million votes cast in 2020, and given that we saw record turnout of 66.6% due to a number of simultaneous crises, it makes perfect sense that the total number of votes cast in 2024 would fall, but not all the way down to 140 million. Between 1984 and 1988, the popular vote dropped by 1 million. Between 1992 and 1996, the popular vote dropped by 8 million. Between 2008 and 2012, the popular vote dropped by 2 million. No polls seem to indicate an 18 million dropoff in voter turnout this year; that would be unprecedented. According to the line of best fit, it is much more likely that we'll see ballpark 150 million votes cast, probably closer to 151 million. That would indicate a voter turnout of around 62%, which is higher than average but more realistic.
Since 1972, the two major parties combine to win an average of 95.5% of the popular vote per election. Third party candidates do well in waves, with considerable showings (over 5%) in 1980, 1992, 1996, and 2016. Since Kennedy dropped out of the race and neither the Libertarians nor the Greens are making waves as spoilers this year, we can assume that the Democrats and Republicans will have a better than average showing. Looking at the trend lines for major and third party percentages, it would show Kamala Harris receiving 51.7% of the vote (about 78 million), Donald Trump receiving 45.0% (about 68 million), and third party candidates combining for 3.3% (about 5 million).
I do not believe that the actual final results will be anywhere close to this prediction. I think that these extrapolations show Kamala Harris overperforming by a considerable amount. She will almost certainly win the popular vote, I don't doubt that, but I think she will win a plurality instead of a majority (under 50%). Donald Trump is not wildly more or less popular than he was the last two times, in which he received 46.1% and 46.8% of the vote, so I do not think it is unreasonable to assume he will receive a minimum of 47% this year, given demographic shifts among suburban voters, black men and Hispanics. He may even perform better than that, but not better than Harris. I would bet my life on that fact. Democrats have won the popular vote in 7 of the last 8 elections, and polls do not show Trump magically coming up from behind on his third try.
When we exclude years with exceptionally high third party turnout, the major parties average 98.3% instead of 95.5% since 1972, but we can't just pick and choose what data to include. At this point, I'm deviating from the hard numbers and am operating more on vibes. Third party turnout will be lower than in 2016 (5.7%), but probably higher than in 2020 (1.8%). If we take the 3.3% figure from the trend lines, that would mean Harris and Trump would receive a combined 96.7% (146 million votes). If we assume Trump has a floor of 47% support (about 71 million votes), then the best Harris could do is 49.7% (about 75 million votes). There is no conceivable scenario where Trump wins the popular vote, so his ceiling (and Harris's floor) is something like 48.35% (73 million votes each).
The only president to win two non-consecutive terms was Grover Cleveland, but he actually won the popular vote all three times he ran, 1884 (48.8%), 1888 (48.6%, lost the electoral college), and 1892 (46.0%). Trump is the exact opposite, having NEVER won the popular vote but still won the electoral college regardless. Franklin Roosevelt won four times in a row, 1932 (57.4%), 1936 (60.8%), 1940 (54.7%), and 1944 (53.4%). There was a major dropoff between his second and third bid, even though he was ridiculously popular. Trump is NOT ridiculously popular, but I don't think he's going to perform worse this time. Cleveland performed worse because there was a significant third party challenger, and FDR was the first sitting incumbent to run for a third term (at a time when WW2 was ramping up and voters were afraid of America joining). Trump does not have either of these disadvantages, so while it's possible he could perform worse than in 2020, I think Harris has so much baggage attached to her name (real and imaginary) that Trump will do better than he did against Clinton or Biden. A lot of voters are fed up with him, but that doesn't mean they will support Harris.
I have no real data to confirm these confidence intervals, but this what I would expect next month
47.0% to 48.3% for Trump (71 million to 73 million), I'd say 48.0% (72.5 million)
48.4% to 49.7% for Harris (73 million to 75 million), I'd say 48.7% (73.5 million)
Now, the electoral college is anybody's guess! Who fuckin knows?