The Obama campaign had prepared lines of attack directed at the two men thought to be the most likely Republican vice-presidential picks — former Gov. Mitt Romney of Massachusetts and Gov. Tim Pawlenty of Minnesota — but had not considered Ms. Palin a likely enough choice to do the same for her.

For weeks, Senator McCain had wanted to name independent Senator Joseph I. Lieberman of Connecticut as his running mate. But the outrage from Christian conservatives over the possibility that the Republican ticket would be filled out with a supporter of abortion rights became too intense to be ignored.