From what can best be currently determined from existing scholarship, the first former First Lady to attend a national convention was Edith Roosevelt.
She joined her husband former President Theodore Roosevelt at the 1912 Republican Convention in Chicago, Illinois as he fought against the odds to gain his party’s presidential nomination away from the incumbent President William Howard Taft.
When her husband failed to win the nomination, Edith Roosevelt expressed disgust with the intra-party fighting which continued. She chose not to accompany her husband two months later to the national convention of his newly-formed breakaway Progressive Party, which nominated him.
“I have lived, most reluctantly,” she said, “through one party split.”
Although she was the niece of incumbent First Lady Caroline Harrison and lived in the White House while serving as her secretary and assistant, Mary Lord Dimmock went on to marry the widowed, former President Benjamin Harrison four years after her aunt’s 1892 death.
While her status as a presidential spouse was ambivalent, her status as a presidential widow was not.
Recognized with honors as a member of a former presidential family, she attended the 1916 Republican National Convention which nominated her friend Charles Warren Fairbanks of Indiana as the vice presidential candidate.
At the 1928 Democratic National Convention in Houston, Texas, former First Lady Edith Wilson assumed a role of high visibility. With the death of her husband, former President Woodrow Wilson four years earlier, Edith Wilson developed a public profile as the symbol of him, his Administration and his ideals, particularly his failed dream of a League of Nations.
She proceeded to Houston from Washington in the private train care of the former U.S. Ambassador to France Hugh Wallace and his wife, joined by her friend financier Bernard Baruch. In Houston, she was the houseguest of famed financier and Democratic Party Jesse Jones.
She later recalled an unexpected incident which led to a precedent – of sorts, during one of the convention’s evening sessions: “Mr. Jones took me up on the platform. Then without any warning to me at all he stepped on to the rostrum and announced to the delegates, ‘Mrs. Woodrow Wilson will now address you!’”
Speaking in what was described as a “low and resonant” voice, she merely acknowledged the applause. Her voice was evidently not recorded nor was there a transcription of what she said beyond thanking the delegate welcome for her. Edith Wilson did not recall exactly what her words were, recalling only, “I said what I could.”
Four years later, Edith Wilson attended the National Democratic Convention in Chicago, accompanied by Bernard Baruch, where they were joined by writer Clare Booth, who was then a Democrat.
When former First Lady Nellie Taft attended the 1940 National Republican Convention in Philadelphia, she willingly posed for photographs and spoke to reporters in a hotel suite serving as the campaign headquarters of her son, U.S. Senator Robert Taft, who was seeking his party’s nomination.
Although the nomination went to Wendell Wilkie, Mrs. Taft remained seated in the convention hall with her daughter Helene Taft Manning, taking in the proceedings to the end.
It was not so much as a symbol of her late husband but as a powerful political broker in her own right which led former First Lady Eleanor Roosevelt to attend three successive National Democratic Conventions, in 1952, 1956 and 1960.
Although she skipped the 1948 National Democratic Convention in light of the fact that she was then a U.S. representative of the newly-formed United Nations, her presence at the 1952 convention was intended to help Adlai Stevenson win the party’s nomination.
Stevenson’s candidacy being opposed by incumbent President Harry Truman who supported Averell Harriman.
Stevenson won not only the 1952 nomination but did so again in 1956, crediting Mrs. Roosevelt’s support as a key factor in his success, despite losing the general election both times.
Former First Lady Bess Truman did not accompany her husband, former President Harry S. Truman to the 1960 Democratic National Convention in Los Angeles.
Four years later, however, former First Lady Mamie Eisenhower did go with her husband to the 1964 Republican National Convention, held in San Francisco.