Steve Benen argues today that Ronald Reagan wouldn't be able to win the Republican nomination today, because today's GOP has swerved so far to the right that many of Reagan's policies (arms control treaties, raising the debt limit, tax increases, and more) would leave him as a moderate RINO. And he's right about the issues (and, indeed, Reagan frequently took hits from his right while in office; it was probably closely comparable to what Barack Obama receives from his left these days).
However, while a good piece of rhetoric, that sort of analysis mistakes, I think, the way that issues and ideology actually work. It's not that Ronald Reagan was seen as a conservative icon in 1980 and won the nomination because he rigorously held conservative positions on all issues; it's that Reagan won the nomination because he was a conservative icon, and people who thought of themselves as conservatives adopted his issue positions because he held them. That's easiest to see on taxes and budget deficits. Up until the late 1970s, standard conservative orthodoxy held that budget deficits were Bad Indeed -- and it wasn't just lip service. They actually cared about deficits, so much so that they opposed Democratic tax cuts and Democratic defense budget increases. When Reagan adopted supply-side policies, that all changed (despite the plain fact that Reagan did actually accept various tax increases), and tax cutting became the new conservative orthodoxy.
Of course, there are always dissenters, even when a Reagan-like figure happens to be around (Ted Kennedy played a similar role on the liberal side). But very little of that criticism from real ideologues filters down to rank-and-file voters, or to most of the more practical party actors. Unless, that is, they're looking for an excuse to distance themselves anyway.
What's happening now is twofold. First, in the absence of a Reagan-like figure, what counts as "conservative" is up for grabs. And, at the same time, the particular set of presidential candidates in both the 2008 and 2012 cycles have reinforced the confusion, because all of them have entered the contests with serious blemishes on their record of adhering to conservative orthodoxy (other than a few safely conservative candidates, such as Rick Santorum this time around, who have other problems that keep them from being plausible nominees).
Since Reagan didn't have that problem, he'd mop the floor with this group. And in doing so he'd let the party know what being conservative meant, and pretty much everyone on the Republican side would go along with that.