Going into this election John McCain had to try to get votes from 4 different groups.
1) The woman voters
2) The evangelical conservatives
3) The moderates and independents
4) any disgruntled Hillary voters
By choosing Sarah Palin, he automatically locks up the conservative base. They see that he is willing to choose a woman with strong conservative views, and also a woman who has executive experience, running a state with a great number of state employees, and being responsible for a huge budget. She did an exemplary job as governor.
Now given the situation with her daughters pregnancy, Sarah Palin may choose to step aside, if her daughter asks her to, since her family would come first.
This allows McCain to choose a more 'middle of the road' candidate, and if the Dems attack that candidate too, it will look like they are attacking any candidate, regardless of their qualifications. ( see Bork )
If McCain then chooses a Lieberman ( or someone more toward the conservative right )
McCain can say to each group, he tried.
To the Hillary women, and the woman voters: I tried to nominate a very qualified woman, but, you see how the sexist Democrats treated her. I would have loved for her to be my Vice President, but, SHE chose to step down, doing it for her family.
To the Evangelicals.....he can state the same facts, he wanted Sarah Palin, but, she chose to step down
To the moderates/ Independents: He then chooses a Lieberman ( or someone less far right as Palin ) and he may get their votes too.
JOHN McCAIN....MASTER STRATEGIST