The conventional wisdom just under a month from election day is that Republicans are poised to hold or even expand their Senate majority, even as they likely give up the House majority. Conventional wisdom is often wrong, but all available evidence based on the polling seems to suggest it’s on target at the moment. It’s possible, of course, that there is a massive wave of Democratic votes that is being undercounted by traditional polling methods, but it would be unwise to stake serious predictions on it.
The reason for the disparity between the House and Senate situation is pretty simple: despite the gerrymandering of districts, a House election comprises the entire American electorate while the one-third of Senators currently up for a vote does not. Of particular interest in the House are those districts that contain the diverse, better educated, mostly suburban populations disgusted with Trump and rapidly fleeing the Republican Party–as well as a smattering of more rural, previously Democratic districts (particularly in the Midwest and the Rust Belt) that seem to be second-guessing their votes for Trump.