Plains, Georgia - Fifteen days after turning 100, former president Jimmy Carter cast his vote in the US election on Wednesday, fulfilling an earlier declared wish to live long enough to back Kamala Harris in the poll.

The former Democratic leader "voted by mail," according to the Carter Center, the non-profit he founded after he left the White House in 1981 to pursue his vision of world diplomacy.

The centenarian – who left office under a cloud of unpopularity, but has seen his star rise ever since – took advantage of early voting in his home state of Georgia, where he is receiving hospice care.

Carter had told his family earlier this year that living long enough to vote for Harris and help defeat her Republican rival, Donald Trump, was more important to him than his centennial, according to the Atlanta Journal-Constitution newspaper.

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In the end, he reached both milestones.

More than 420,000 people have cast their ballot since early voting began Tuesday in Georgia, according to Gabriel Sterling, a state election official who posted the figures at midday.

Election Day is November 5.

Carter, a one-term president, has been receiving end-of-life care in his hometown of Plains in Georgia since February last year.

He is the first-ever former president to reach the century mark, another extraordinary milestone for the one-time peanut farmer who worked his way to the White House.