Ethel Kennedy, the widow of Senator Robert F. Kennedy and a popular and vital force in the Kennedy political dynasty, died on Thursday. She was 96.

Her grandson Joseph P. Kennedy III announced the death on the social media site X, giving the cause as complications of a stroke she had last week. He did not say where she died.

Her death came a little more than six weeks after her third eldest child, Robert F. Kennedy Jr., ended his long-shot independent presidential campaign and endorsed former President Donald J. Trump in his bid for re-election.

Mr. Kennedy’s decision to support the Republican nominee and his earlier choice to challenge Mr. Trump’s Democratic rivals, initially President Biden and then Vice President Kamala Harris, caused a painful breach in the Kennedy family, compelling some of his many siblings, cousins, nieces and nephews — heirs to a staunchly Democratic lineage — to speak out in dismay and anger and originally endorse the president, a friend of the family, over Mr. Kennedy.

Mrs. Kennedy’s passion for politics was so consuming that she was often said to be “more Kennedy than the Kennedys.” Displaying energy and humor, she campaigned tirelessly for her husband and other Kennedys, much of the time while pregnant.

Her 11th and last child was born after her husband’s assassination in 1968 in Los Angeles, as he campaigned for the Democratic presidential nomination.