I’ve read that every time the House has flipped, so did the Senate. I haven’t researched that myself beyond having read it more than once, but is does seem logical. If someone goes into the booth to vote a Democrat out of the House, they are probably going to vote for a Republican Senator if there is one on the ballot.
I’ve noticed something interesting this cycle. Precious few campaign signs have party affiliation on them. If there is part affiliation, it is Republican. I can’t recall seeing a single sign with a Donkey on it.
The reason the Senate would stay Dem is high turnout in urban districts whose House district is Dem regardless. The high turnout in a particular inner-city district won’t change the balance in the House but could tip a statewide Senate seat.