I should warn, however, that the polls in many key races are close enough that these predictions aren't really worth much. A completely "normal" polling error of 2-3 points could make an enormous difference. And I have little confidence in the "gut feel" reactions of pundits, whose error rates are far higher than those of polls:
With the huge caveats noted above, I think the Republicans will take the Senate with a 51-49 margin (they will win Nevada and Georgia, while Ds take Pennsylvania and all other seats remain in their current configuration), and the GOP will also take the House, probably with gains of around 20-25 seats.
The GOP takeover of the House is very likely (probably at least 80%). By contrast, the Senate situation is highly uncertain, because there are multiple very close races, including some where candidate quality is a big issue for the Republicans. Outcomes ranging from 52-48 for the Dems to 53-54 seats for the GOP are all at least somewhat plausible.
Barring a really big polling error going against the GOP, the above range of outcomes is still far less of a GOP gain than we would expect in an off-year election (which inherently favors the "out" party), where we also have a 40 year high in inflation and a crime wave. The Republicans are likely paying a price for the combination of low quality candidates, the Dobbs decision, and Trump.
There are those who believe polls are always or almost always biased in favor of the Democrats. That did happen in 2016 and 2020. But 2012 (where there was a substantial pro-GOP bias) and 2018 (a small one) prove otherwise. It's possible the polls are overestimating the Dems this year. But it's also possible pollsters overadjusted to correct the mistakes of the last election, leading to errors the other way. At this point, GOP-aligned polls are showing a picture much more favorable to their preferred party than independent polls are. We will soon know which is right (if either is).
As in most elections, short-term economic considerations play a huge role, despite the fact that incumbents generally have little control over them. Voters think they have more leverage than is actually the case.
What do I want to happen? Probably for the Dems to keep the Senate, while losing the House. Having seen the D legislative program of the last 2 years, I don't want most of it to be enacted. A GOP-controlled House will stop that. On the other hand, the GOP deserves punishment for their election denial, Trumpism, and horrible candidates (among other sins). Losing a golden opportunity to take the Senate will achieve that. Also, a D Senate will enable Biden to continue to make and confirm executive and judicial nominees (while some of his nominees are bad, on the whole I think it's better if he can get them through than if large numbers are blocked). Of course, history shows that I rarely get what I want in elections. So the fact that I prefer this outcome may itself be a good reason to predict it won't happen!
Marijuana legalization is on the ballot in several states, and should pass in all of them. Yay! This may be the one unequivocally good thing that happens tomorrow.