President Obama isn't on the ballot this fall.

But, unless something drastic changes between now and November, Obama's problematic poll numbers will complicate -- but not doom -- his party hopes of holding onto its Senate majority. That's because one of the strongest predictors of seat gains (or losses) in midterm elections is the job approval numbers of the sitting President. The better his numbers, the better his party does -- or, more accurately, the less worse his party does -- in the fight for control of Congress. The worst his numbers, well, you get the idea.

This chart, put together by Fix friend Bruce Mehlman, a lobbyist with the firm Mehlman Castagnetti Rosen Bingel & Thomas, tells that story. (The numbers Mehlman uses for presidential approval are from Gallup.)

Image courtesy of Bruce Mehlman

Put simply: Once a presidents's job approval numbers slip below 50 percent, bad things start happening for his party in Congress.  In the six midterm elections since 1974 where the president's job approval was sub-50 percent, the average seat loss in the House was 39.  In the Senate, the average was  a 4.5 seat loss.

That's bad news for Democrats. But, context matters here. Because of the Republican wave election of 2010 and the subsequent national redistricting process, there simply aren't all that many House seats left for Republicans to logically target.  (There are only 26 seats held by a House member not of the party who carried the district in the 2012 presidential race.) So, because House Democrats have already lost so much, losing more isn't likely.

In the Senate, Democrats would gladly take a four or even a five seat loss (as historical trends suggest they will endure) because it would mean they would retain control of the Senate -- albeit it narrowly.

President Obama will be a drag on Democrats this fall. He won't -- at least according to Mehlman's data -- be the anchor that drags his party under.