No, it isn't. There are several reasons beyond the caveats that apply to the Ohio results. Most importantly, while changes in special elections correlate fairly well with the difference in national House votes in an election year, there is no similar correlation at the presidential level.
To assess this, we can use special election results data compiled by DailyKos, specifically the site's Special Elections Index (SEI). The SEI compares special election results not only to the most recent presidential election, but also to other recent elections in the district. Its goal is to measure changes in electoral contests while controlling for the effects of particularly dramatic election cycles. For example, many results may look relatively Republican when compared to Barack Obama's success in the 2008 election.
What DailyKos found is that their index correlates well with the House vote share. You can see this in the chart below. The diagonal dashed line shows that the SEI and the national House vote share are perfectly aligned. That is, the SEI is exactly the same as the vote share. That is, the years closer to the line were when the relationship between these indicators was closer.
And with a few exceptions, most years are pretty close to that line.
Incidentally, an election where the margin wasn't that close is 2022. This is one of two elections where the SEI favored Democrats and the House results favored Republicans.
The question of why the difference exists is a fundamental one. It seems to be due to differences in voter turnout. Midterm elections don't attract as many voters as presidential elections, but they generally attract more voters than special elections, the latter of which have been fueled by Democratic indignation over election results. Roe v. Wade By November 2022, that anger had been absorbed to some extent by the broader electorate.
That question will remain until November: To what extent will the energy Democrats enjoyed in the special election carry over to a presidential election with a larger electorate? This correlates with another question that's harder to answer: How will voter turnout in 2024 compare to the record turnout in 2020? Might low turnout among disinterested voters favor candidates supported by the most motivated voters? (Early indications suggest that it might.)
But we shouldn't assume that the SEI value (still undecided for 2024, but favoring Democrats) tells us anything about the difference in vote share in presidential elections. Since 1992, the correlation between presidential election outcomes and the SEI has been much lower than the correlation between House election outcomes.
Part of this is down to 1996 being an outlier year: Republicans performed well in a special election, but President Bill Clinton was easily re-elected. If we remove that year, the correlation is a bit stronger, but not as strong as the House results. This may be due in part to fewer comparable elections, but not solely to them. Of course, adding more elections could easily weaken the correlation. There is no reason why we can't have another year like 1996.
The simplest assumption here is also the least plausible: we simply don’t know if the results of the special election will tell us much about November. As we’ve noted many times before, this is an unusual election under unusual circumstances. Perhaps this is why the SEI doesn’t correlate well with the presidential election or with political events in and of itself.
For those who see the special election results as a reason for optimism about Biden's chances of reelection, there is one useful thing to note: There have been examples of special election results compared to national election results in the current climate, and those were in 2022, where the special election results overstated how Democrats would fare nationally.
