Lucy Rutherford

In 1914, before he was stricken with polio, FDR fell in love with his wife's secretary, Lucy Mercer. 

Eleanor found out about it and the two decided to divorce.

FDR's mother told them that if they divorce, he will forfeit his inheritance

Franklin and Eleanor agreed to stay married, but as political partners, not lovers; Eleanor asked that Franklin forsake Lucy, which he agreed to

Franklin did not forsake Lucy. He purchased a cabin in central Georgia where the two could routinely rendezvous.

Eleanor apparently also developed a romantic relationship with Lorena Hickok, a journalist.

The affair went on for the rest of his life. On the day he died in Georgia, Lucy was there with him, having gifted him with a professional artist to paint his portrait.

When Eleanor found out that Lucy had been with him so often in Georgia, she had a fit.