McCain had respectability across the board. Albeit he did end up squandering it. He was a moderate, had an honorable service record, could work with others (there's a word for that which I can't recall given we've had nearly eight years of a Washington militantly without it), and enticed Republicans, Independents and Democrats. He was seen as who we should have had instead of Bush for most of those eight years of the Bush administration. The part where he squandered it was by going hard to the Right to win the election and totally undermining all that.
The thing about 2008 is that any Republican that years is like a Republican running in 1928. It doesn't matter who ran, nor how respectable a candidate. The Democrat is going to win thoroughly; it was totally an issue of whoever won the Democratic nomination was (but maybe not) going to win the presidency. And this was a time when even the people who would become Tea Party people would support Obama or Clinton (before the news/political sphere made everyone think they were living in some kind of movie version of reality). Not to say the seed of that type of Conservative movement wasn't already there; I recall people losing their minds, and "redistributing the wealth", "Socialist" and talking about "he's gonna be shot anyway" and all that. But it wasn't so loud and widespread and serious.
Romney honestly had none of whatever McCain had. He won't inspire the base, and he'll be seen as a sign of what was wrong at the moment; the extremely rich who tanked the economy, and how the Republican party is in that camp. That is what the election will become. You can cry "Socialist" all you want, and it will stick because that whole socio-political sphere is what it is in America (which is a hell of a lot of people who have opinions but not the cognition to properly form them) but that image of Romney as ultra-wealthy, privileged and out of touch is going to stick. And what is Romney going to say? He can't connect with the concerns of America in 2008, which is the economy and a major recession. He can't connect to people worried about their jobs and their economic well beings, or who already lost their jobs. He has to and will fail at doing so miserably.