With Democrats easily winning most special elections lately one might

[EarlG]

3. Some Republicans will vote for Dems, but

I think the more likely effect you're seeing is that over the last ten years Trump's MAGA brand has activated a lot of voters who weren't always reliable voters, and those people came out to vote for Trump in presidential elections -- but many of them don't give a crap about the Republican Party generally, or their own local elected officials. Whether Trump is on the ballot or not has a big effect.

Meanwhile, the party which is not in power always has more active voters -- the president's party usually always loses seats in mid-term elections. So it's a combination of fairweather Trump voters not showing up, general voter disapproval of Republican policies, plus Dem overperformance because Democratic voters are considerably more outraged about what's going on in the country and see voting as a way to stop it, and are therefore turning out in greater numbers than usual.