Aides to the California Democrat confirmed the timing and said Governor Gavin Newsom was aware of her decision, clearing the way for him to appoint fellow Democrat Alex Padilla, now California's secretary of state, to serve the final two years of Harris's term.

Padilla will be the first Latino senator from California, where about 40 per cent of residents are Hispanic. Newsom announced his choice in December, following intense lobbying for the rare Senate vacancy from the nation's most populous state.

Harris will give no farewell Senate floor speech. The Senate is not scheduled to reconvene until Tuesday, the day before Inauguration Day.

Padilla's arrival, along with Harris becoming the Senate's presiding officer when she is sworn-in as vice president, is part of the Democrats' upcoming Senate majority.

But the party still needs senators-elect Jon Ossoff and Raphael Warnock of Georgia to be certified as victors in their January 5 elections and then be sworn in.

Harris's early departure from the Senate has multiple precedents.

Biden was the last sitting senator to be elected vice president. He resigned his Delaware post on January 15 2009, five days before he and Barack Obama were inaugurated.

Obama, a senator at the time of his election, had resigned his Illinois seat two months before Biden.

Australian Associated Press