It might seem counterintuitive, but losing an election doesn’t mean you weren’t “electable”.
In 2012, one of the main arguments for Mitt Romney as the Republican nominee was that he was the most electable. This point is usually supported by favorability polls and subtle campaign factors like wide, not specific or tribalist, general election appeal.
Romney’s claim of electability in 2012 was based on this data. His favorability varied quite a bit, but was positive from early summer right up to election day. More people liked him than didn’t, in the end. Unfortunately Obama had a comparable favorability rating; he wasn’t the unpopular figure most Republicans assumed he was.
As we all know, Romney lost. He wasn’t as electable as he thought, but he was still the most electable of the Republican candidates at the time. Rick Santorum’s favorability rating was almost never in positive territory. Newt Gingrich was one of the least popular politicians in the country, two weeks after he won the South Carolina primary.
This year the electability argument has come back around again, primarily as a point in favor of Marco Rubio. As a young, well-spoken conservative with minority immigrant parents, a middle-class history, and solid grasp of current cultural trends, his appeal is broader than the Republican party has seen in ages.