Campaign contributions can provide a look into both voter enthusiasm and a campaign's ability to get its message out in the final days of a race. As in the past, much of the money is coming from out of state.

Still, "having more money is not always predictive of success," according to Kyle Kondik, managing editor of nonpartisan election analysis site Sabato's Crystal Ball.

"This year, a lot of those challengers are outraising the incumbents," Kondik said in an email. "It's hard to quantify how much the money matters but many Democrats are going to have the resources to get their message out in the closing days."

As Democrats try to flip a net 23 GOP-held seats and take a majority, most of the key battles this year will take place in districts held by Republicans. In those races, a money advantage will not necessarily lead to more votes for Democratic candidates.

Cook shows 17 districts of those potential flips leaning toward Democrats. They out-raised their GOP rivals and also hold a cash advantage in 13 of those races.