As we wait for returns to come in Tuesday evening in the Republican primary for U.S. Senate in Mississippi, here's a data point to chew on: Outside groups have spent a whopping $8 million in the campaign. That's about double what both the candidates spent. Yes, combined.

Most of the money went toward backing state Sen. Chris McDaniel over incumbent U.S. Sen. Thad Cochran. According to a Sunlight Foundation tally of spending on the race that must be reported to the Federal Election Commission, third-party groups spent nearly $5.3 million on behalf of the tea party challenger. Cochran's allies spent nearly $2.8 million.

When it came to candidate spending, McDaniel was no match for Cochran. The incumbent spent about $3 million, compared to $1 million for McDaniel.

The biggest spender overall was the Club for Growth, an anti-tax group that backed McDaniel from the outset of his campaign. The Club spent about $2.5 million.

The biggest pro-Cochran spender was Mississippi Conservatives. The super PAC designed specifically to help the senator dished out nearly $1.7 million on the campaign.

Polls showed a razor-tight race between Cochran and McDaniel headed into the day. It's also possible that we won't know the winner by this time Wednesday. With a third candidate in the race, Cochran and McDaniel could each fall short of winning a majority, thereby triggering a runoff later this month.

So, this might not be the end of the spending barrage.