I can't believe this hasn't been discussed.
If Kerry becomes President, Mitt Romney gets to appoint (probably Bill Weld) to Kerry's seat. While Kerry could take out Weld as the incumbent, I bet Weld could squeak out a win if he were the incumbent.
Why is this so important?
The next Dem President will, if at all possible, need the Senate. And we need 2 seats to get that 50/50 tie.
2 is possible. 3 gets hard.
For that reason alone-- we should keep Kerry in the Senate.