At his popular election blog FiveThirtyEight.com, Nate Silver says Sen. Charles Grassley’s seat would most likely fall to a Democrat if he retires in 2010. Otherwise, he’s a safe bet for re-election.

The prediction came during his periodic series ranking the seats in the senate currently known to be contested in 2010. Grassley’s ranks 17 out of 35 contested races based solely on his age.

17. Iowa (R-Grassley)

Approval/Favorability Ratings: Chuck Grassley (R)
Poll               Date    Approve   Disapprove    Net
SurveyUSA          10/18     63          29        +34
Selzer             2/19      67          18        +49

This race ranks where it does solely because Grassley will be 77 in 2010 and could retire, in which case the race probably leans Democrat. Absent a retirement, a kamikaze mission by someone like Tom Vilsack against the popular incumbent is unlikely to succeed.