If Donald Trump wins the Republican nomination, he will be an untested nominee in one sense and is likely to be an untested nominee in a second sense as well. The first sense is the one that Erick Erickson brought out in his contribution to NR’s anti-Trump symposium: He is a recent convert to conservatism; we have never seen him stick with any conservative view when it was not in his immediate interest to do so. If he were president–or even the nominee–he might well abandon many of the conservative positions he has lately adopted. This is of course a risk with any candidate, but it is a higher one with someone who has not invested much of his reputation in conservatism.
The second sense in which he would likely be an untested nominee is that he would have gotten it without ever facing a serious ad campaign against him. Were he the nominee, we can trust that the Democrats would not be as reluctant to put money into taking him down as Republicans have been. And his numbers with the national electorate are already weak even in the absence of a sustained anti-Trump campaign.