U.S. Sen. Thad Cochran (R-Miss.) speaks on stage during a pre-election day rally at the Mississippi Agriculture and Forestry Museum in Jackson, Miss., on Monday. (AP Photo/The Clarion-Ledger, Joe Ellis)

It's not official, but Sen. Thad Cochran (R-Miss.) is very likely headed to a three-week runoff with state Sen. Chris McDaniel (R).

And while the race is close enough that anything can happen -- it's currently McDaniel 49.5 percent to Cochran 48.9 percent -- history suggests Cochran faces a tough path.

That's because runoffs are very unkind to incumbents.

We looked at seven times since 2000 in which an incumbent has faced a primary runoff, and then compared the primary vote totals to the runoff totals in each race. In all but two cases, the challenger took at least 75 percent off the votes that were "up for grabs" in the runoff, and in two of those cases, the incumbent actually lost votes in the runoff.

(We're including six congressional races and also added Texas Lt. Gov. David Dewhurst's runoff loss this year -- because examples of incumbents facing runoffs is so rare and we wanted more data.)

Here's how that looks, chartified:

One of the exceptions to the rule was Rep. William Jefferson (D-La.), who actually lapped his opponent in his 2008 primary runoff. That was a highly unusual set of circumstances, of course, as Jefferson was under federal indictment. He also happened to be running in a primary runoff on the same day as President Obama's historic election as the first black president. Jefferson was the lone African American candidate in the runoff in heavily black New Orleans.

Just a month later, Jefferson inexplicably lost to a Republican in one of the most Democratic districts in the country. Clearly, this had a lot to do with Obama.

The other exception was Sen. Blanche Lincoln (D-Ark.), who in 2010 got a big dose of help from national Democrats concerned with keeping her seat in Democratic hands. She won the runoff -- taking more "up for grabs" voters than her opponent, Lt. Gov. Bill Halter -- but lost big in the general election.

The rest of the seven races above demonstrate the idea that the primary is generally all about the incumbent, and if someone doesn't vote for the incumbent in the primary, it's unlikely they'll be wooed in that direction in the runoff.

On top of that, turnout in runoffs is often much lower, which generally leads to a more ideologically polarized electorate. More motivated voters tend to be more conservative in GOP primaries, which again hurts Cochran.

Now, this data shouldn't be oversold. After all, there are less than 2 percentage points worth of votes up for grabs -- far less than in any of the examples above. And all Cochran needs to do to get to 50 percent plus one is gain a little more than 1 percentage point. That's certainly doable, and Lincoln's example should be heartening, especially if the national GOP rallies for Cochran.

But if history is any guide, those third-candidate voters will favor McDaniel, as will the turnout in the runoff -- which is likely an even bigger factor.