As to whether Bush has been derelict in promoting Republican Presidential prospects in 2008, I've believed it since the day Bush picked Dick Cheney as his running mate in 2000 (as much as I admire the latter, but for his promotion of LOST).
No, it's not. If Frist gives up his seat, his replacement in the Senate would be appointed by a Democratic governor.
Well, Bush doesn't want a Republican to win in 2008, as it would effectively lock the door to a Jeb presidency. That's why they have been courting the Clintons so heavily. The same reason the Clintons sabotaged Kerry in 2004.