The short issue is McCain can't. He comes off in the narrative of history, at least to me, as part the sacrificial lamb, part the old dog being given his last chance.
The Republicans were shattered in 2008. They were shattered over every jagged rock that was the failure and unpopularity of the Bush administration. They were divided, they were unpopular. We should all be able to remember those days. In that period and the early years of the Obama administration, there was a growing consensus that the Republicans were out of it for the near future and that this was possibly a new political age. The latter had truth to it, but the former proved wrong as the Republicans went to the right and surged back with vengeance. That doesn't really matter for the topic, though, but it does show the thinking of that era which should reflect the way the Republicans had lost before they even started for 2008.
McCain won the nomination because he was, in the wake of the Bush administration's total unpopularity, the greatest Republican, and he was the elder Republican. And I contend that he, like Goldwater in 1964, was put out there as a sacrificial lamb; someone with support who was not necessarily ideologically popular with the thinking of the party at large, who was allowed the nomination because his time had come but that time was also the worst moment for it to come, who was going to lose as a result.
And he lost heavily. Had it not been for Palin firing up Conservatives, he likely would have lost even more.
You are not going to get McCain to the White House under anything like the situation as it was. The only way I've seen McCain win 2008 in a plausible way is in a timeline where John Edwards was nominated by the Democrats, has his scandals break, and McCain manages to get the win on that.