In 1940, he was dark-horse candidate Wendell Willkie's running mate. "Although Willkie’s victory [at the Republican convention] showed that he was adept at convention politics, it was clear that he had given little, if any, thought to a post-nomination strategy," writes Timothy D. Walker, Willkie's grandnephew, of the convention. "This was understandable, of course, because all of Willkie’s efforts had necessarily been directed to overcoming tremendous odds and capturing the nomination itself. The first evidence that Willkie had no post-nomination strategy was his complete lack of thought into the question of who to select as his running mate. For advice on this matter, Willkie turned to Joe Martin, chairman of the convention. Martin suggested Oregon Senator Charles McNary, which Willkie readily agreed to even though McNary had been in the group of GOP leaders who had issued the 'Stop Willkie' statement on the first day of the convention.
"McNary agreed to be the candidate for Vice President, but not because he was a Willkie fan. Rather, he accepted the offer out of his loyalty to the Republican party. It was a strange pairing, because Willkie and McNary had conflicting views on major political issues. McNary was an isolationist and a supporter of public ownership of electrical power. But the union was also very practical; it was thought that McNary’s presence on the ticket would counterbalance Willkie’s major liability: his close connection to East Coast financiers. In any case, the combination worried FDR, who confided to his staffers on the day after the nomination that the Republicans had nominated a very strong ticket."
In the end, it wasn't a strong enough ticket to unseat FDR in 1940, who won his third term by about 5 million popular votes and 449 to 82 electoral votes.