Well, he got the GOP nomination, but he didn't 'earn' it. 
I'm not sure if he was unopposed or not.
Hoover was unopposed in 1932, there were only 4 votes cast against him at the convention. However, it was more the case that no one wanted to run as a Republican that year. Most everyone could clearly see the writing on the wall.
Roosevelt did have a majority of Democratic delegates in part due to success in those states that had primaries. The Democrats required a 2/3 majority rule back then, so FDR could not get a first-ballot nomination. Some suggest that the move to name Garner of Texas as VP secured his nomination (4th ballot).
