A chart that will make Senate Democrats wince

President Obama made clear on Thursday that even though he isn't on the ballot on Nov. 4, his policies very much are.

Voluntarily nationalizing the midterm elections as an unpopular president is a strategy sure to make many Democratic Senate candidates very unhappy. Why? Because there is a very direct historical correlation between presidential approval and how that president's party does in midterm elections.  Bruce Mehlman, a Republican lobbyist at the firm of Mehlman Castagnetti, makes the point perfectly with a chart included in his latest quarterly political forecast, which he shared with me on Friday.

Here's Bruce's chart.

Remember that to win the Senate majority in 32 days, Republicans need to net six seats -- right where history suggests they'll be if Obama's approval stays close to where it is today. And also remember that there are seven Democratic-held Senate seats in states that Obama lost in 2012 -- and where his numbers have only fallen since.

That sound you hear is Senate Democrats groaning.