I'm not exactly sure that Mitt Romney's father was a Mormon.
Mitt Romney may have converted from his parents religion.
George Romney was a Mormon.
From an American Motors history website: http://www.amxfiles.com/amc/romney.html
"A lifelong member and former bishop of the Church of Jesus Christ of Latter-day Saints, Mr. Romney spent two years as a Mormon missionary in England and Scotland during the 1920s. Friends said there was an evangelical strain about many of his undertakings later in life as well."
I knew that about him but, as I said, I don't remembering his religion being an issue back then.
From George W. Romney's 1995 obituary:
"A lifelong member and former bishop of the Church of Jesus Christ of Latter-day Saints, Mr. Romney spent two years as a Mormon missionary in England and Scotland during the 1920s."