He started running for POTUS the second he was sworn into the Senate.
And I am in his state; he wasn't going to win re-election for the Senate anyway. His own town, you know -- the milltown -- went sold Bush/Cheney in 2000.
He's been in Iowa more than he's been here. Or in the Senate, for that matter.
As I said above, if they were to be elected, he'll be running from the second he's sworn in as VP.
Don't forget New Hampshire where he spent a good part of the year leading up to the primaries, meeting "regular" folk.