From Amy Sullivan's very good piece on pro-choice women and McCain: 

But McCain's more traditional abortion rhetoric is leavened by his carefully maintained political brand as a "maverick" politician. Nancy Keenan, president of NARAL, believes that has led many voters to make incorrect assumptions about McCain's views on abortion and is one reason he is now courting pro-choice women, particularly Hillary Clinton's supporters. "People think that he's a maverick and that must mean that he's a moderate," Keenan says. "And they come to the conclusion that if you're a moderate, you must be pro-choice."

Exactly. The real problem here, though, is that the term "moderate" has been defined by the press as someone who espouses fiscal conservatism and social liberalism. A Republican who was extremely socially conservative but relatively liberal on economic issues would never get the label. Moreover, Keenan makes a good point about the connection in people's minds between a "mavericks" and a "moderate." And--bingo--McCain gets a freer ride than he otherwise would.

--Isaac Chotiner