John McCain = George Bush in 1992
This election is unique in modern times. Neither the president nor the sitting vice president is running. The closest thing to it in modern times is 1952, Ike vs Adlai Stevenson. But Stevenson was a reluctant nominee, who was drafted by the party bosses despite Kefauver having won 12 out of 15 primaries. Behind on the first ballot, he won the third, the last time any major pick has gone beyond the first ballot.
Ike actually had a fair case that he won the primaries, but it was close between him and Taft. After some bitter backstage wrangling on his behalf, he won on the first ballot.
So, things were a lot different back then. Lots of smoke-filled rooms at work, and much less regard given to the popular will in picking nominees.
Some things don't repeat, but they rhyme. Truman could have run again, but didn't because of his low approval rating, which unfortunately is not as low as GWB's right now.