Public presumption of a win (very dangerous before it actually happens), and
Public presumption that they saw him as some kind of leader, instead of a useful idiot.
What, winning his own district or winning the majority leadership?
Until recently you would have to say that Murtha was a good bet to win reelection. (Unopposed in '04, winning 74% in '02)...
My sense of it is that there is an assumption that unless there is dissatisfaction with the leadership (or some other circumstance such as the whip losing election, or the whip moving up to Speaker just as the leader retires (Gingrich '95), that the standing whip moves up to leader unless the whip doesn't want the position. Now why would the Democratic caucus be dissatisfied with its leadership if it got majority status?
Take a look at recent leaders:
2003 Pelosi (was whip)
1989 Gephardt (Coelho resigned under scandal)
1987 Foley (was whip)
1977 Wright (whip McFall defeated in home district)
1973 O'Neill (was whip)
1971 Boggs (was whip)
1961 Albert (was whip)
1955 McCormick (was whip)
Hmmm...I see a pattern here. My guess is that Steny Hoyer will be the next leader should the post become available.
This does not mean that Murtha cannot campaign for the slot, although I can understand Pelosi et al. telling him to cool it.
On the Republican side, yes, 2006 Boehner is an exception.
I don't remember Leslie Arends, whip 1955-75. So far as I can figure he never sought the leadership. Halleck, Ford, and Rhodes leapfrogged him in any case.