Of course they do. But the question is whether they are raising the bulk of that money from big business, or from their constituents.
Most of Their constituents do not have as much to offer financially but they do commit very valuable resources such as time and votes. When these folks stop donating it may not impact the overall purse, it won't as a fact because businesses will continue to contribute to a Majority Party as standard practice. What loss of 10 dollars here, and 50 dollars there represents is the loss of a valuable vote and possible grassroots activist so crucial election day.
This isn't about the money. Businesses will donate, special interest groups will donate...they do so for a seat of power. I'd be surprised if the money trailed off. It's sending a signal that what they need most, VOTES, will dry up if they don't follow through.
You're one of the few who doesn't think fundraising is a major measurement of how one's campaign is progressing.