Altogether, Senate candidates challenging incumbents in primaries raised nearly $19 million in 2013 and 2014, according to a review of the most recent campaign finance data from the Federal Election Commission. That's a lot of money; for reference, primary challengers to incumbent senators raised around $22 million over the previous three election cycles (2008, 2010, and 2012) combined.
Even fairly recently, incumbents used to be able to pretty much shut off fundraising for any challengers they got. But now, there are more ways to access money—like the grassroots conservative donor networks that come with endorsements from groups like the Club for Growth or Senate Conservatives Fund.
Messing around in the opposing primary to try and boost a less-electable candidate is a sexy strategy these days—especially after Senate Democrats used it to help win races in two successive elections by promoting self-destructive Republican candidates (Nevada's Sharron Angle and Missouri's Todd Akin). But the party came up empty this year, trying it in a number of House, Senate, and gubernatorial races where the strongest GOP nominee still won, often by a wide margin.
In the end, the attention those two successes got was out of line with the overall effectiveness of the strategy. "Because of the success with Akin, I feel like it's this new trendy thing," one Democratic strategist said. "I've been on calls where people are like, maybe we should screw with their primary. But you shouldn't do it nine times out of 10, or maybe 95 out of 100. It's very easy to get a backlash, and we don't know how to poll hardcore Republicans—as a Democratic pollster, you're just guessing."