Approval of President Obama has fallen to 46% among 18-29 year-olds, a key part of his coalition in 2008, according to a poll by Harvard University’s Institute of Politics.

The survey of 2,028 Americans often described as millennials, taken from Nov. 23-Dec. 3, showed that 51% disapproved of the job the president was doing. Obama’s approval was down 9 percentage points since Harvard’s last survey of this age group in February…

Obama still leads potential challengers among 18- to 29-year-olds, but far from the margins he piled up against McCain. Obama leads Romney 37%-26%; Gingrich and Perry each 39%-23%. But more than a third of millennials said they were undecided.