Today’s primary in Nebraska for the Republican nomination for Senate is important for several reasons, but in particular it may turn out to be a milestone for the current state of campaign finance — if longshot Deb Fischer shocks two statewide elected officials and wins the seat of retiring Ben Nelson.

Until very recently the primary appeared to be a showdown between Nebraska Attorney General Jon Bruning, a seemingly up-and-coming politician who appears to be a very strong general election candidate, and state Treasurer Don Stenberg, who has received support from national conservative groups like the Club for Growth. It’s not unusual for a third candidate to benefit from a vicious negative campaign run by two well-funded frontrunners, but there’s a new element this year: The $200,000 dropped by Chicago Cubs owner Joe Ricketts into the race on behalf of Fischer and against Bruning.

This is exactly the kind of contest that people concerned about the wide-open spending associated with the Citizens United decision should be worried about. Big money isn’t apt to make much of a difference in presidential campaigns, because of diminishing returns and the large amounts of money and press coverage that those races receive. Nor is it likely that big money will matter a lot in Congressional races this November; while it could sway some votes on the margins, party identification, and not advertising, drives most votes in November.

But a Senate primary is exactly where someone should theoretically be able to show up, write a check, and buy himself or herself a Senator. It’s exactly where a late hit — fair or not — could be enough to tilt an election. There’s nothing to keep a pure outsider, someone acting on behalf of an interest group or even someone from the other party, from trying the same tactic. And we’re not talking about all that much money here…sure, enough to restrict this to the very rich, but not enough that it would be a real strain on anyone with a seven-figure income.

Here’s a question: if Deb Fischer does ride this late hit to a Senate nomination but then turns out to be a third rate candidate and costs Republicans the Nebraska Senate seat they were counting on picking up, will anyone within the GOP wonder if maybe some sort of campaign finance limits might not have been a bad thing after all?

UPDATE: A new Public Policy Polling survey finds Fischer leading with 37 percent, with Bruning at 33 percent and Don Stenberg at 17 percent.