I direct your attention to yesterday's edition of The Fix, Cillizza's political blog. He forcefully makes the point that Obama's sinking poll numbers have serious negative consequences for Democrats' outcomes in the 2014 midterm elections.
I find particularly impressive the chart which compares the popularity of the four recent two-term presidents: Reagan, Clinton, Bush and Obama. Reagan and Clinton remained popular after six years. Bush did not and Obama's numbers to date closely track Bush's disappointing numbers.
If present trends continue, Cillizza sees Democrats losing several Senate seats in '14 and more House seats as well. He concludes:
The loss of the Senate majority and a smaller minority in the House after November 2014 would make any attempt to rack up second-term accomplishments before he left office extremely difficult for Obama. Combine that with the reality that Obama’s second term has not exactly been larded with major wins to date and you understand why Obama and his legacy are on the ballot in 2014 — even if his name is not. And that means his poll numbers matter. A lot.