Whenever there's economic turmoil it's usually bad news for the President's party in midterm elections. Indeed, the incredibly low Congressional approval ratings alone paint a pretty gloomy picture for Democrats. According to Gallup, only 20% of poll respondents approved of how Congress was handling its job in its latest poll. If this statistic doesn't improve, how many seats might Democrats lose? It might help to consider recent history.
Gallup provides the following data on midterm elections since 1974:
Eyeing this chart alone indicates that a 20% approval rating would probably be pretty fatal for whichever party holds the White House. But let's take this a step further and scatter plot these points and figure out a line of best fit:
The red diamonds are the data points from the first chart. The green line is the line of best fit (equation shown). The big purple square is 20% approval. According to this relationship, it implies a 40 seat loss for Democrats in the House.
Of course, individual races matter, and the election isn't for four months. But it's hard to imagine that it could go well for Democrats if Congressional approval hovers around 20%. There hasn't been a midterm election since 1974 where Congressional approval was so low.