Harris, who has served as California's junior Democratic senator since 2017, submitted a letter to Governor Gavin Newsom, saying her resignation took effect at noon on Monday.

Newsom later in the day appointed Alex Padilla, California's secretary of state, to serve out the two remaining two years of Harris' term.

In an op-ed published by the San Francisco Chronicle before her resignation, Harris told Californians that "this is not goodbye" and suggested they haven't seen the last of her in the Senate chamber.

"As I resign from the Senate, I am preparing to take an oath that would have me preside over it," Harris wrote.

"As senator-turned-Vice-President Walter Mondale once pointed out, the vice presidency is the only office in our government that 'belongs to both the executive branch and the legislative branch.'"

Harris, who made history as the first black woman to represent the Golden State in the Senate, has the power as vice president to break ties in the upper chamber.

Harris' authority is likely to be key for President-elect Joe Biden as he looks ahead to pushing his legislative agenda through a Senate that has 50 Democrats and 50 Republicans, thanks to the blue sweep of Georgia's double runoff elections earlier this month.

In her op-ed, Harris noted that there have only been 268 tie-breaking votes cast in the Senate by a vice president since the nation's founding.

"I intend to work tirelessly as your vice president, including, if necessary, fulfilling this Constitutional duty," she wrote.

"At the same time, it is my hope that rather than come to the point of a tie, the Senate will instead find common ground and do the work of the American people."

The first test for Harris could come early, as Biden hopes that Congress will quickly consider his proposed coronavirus stimulus package.

The Democratic-controlled House, which approved an even larger relief bill last spring, is likely to pass Biden's plan without much ado.

But Republicans in the Senate have long refused to back any major relief spending, meaning the chamber may end up voting 50-50 on Biden's stimulus measure, at which point Harris could step in, break the tie and bring the bill to Biden's desk.

Australian Associated Press