"I've taken stock and looked at this from every angle, and over the last few days have come to one of the hardest decisions of my life," Harris said in an email to supporters. "My campaign for president simply doesn't have the financial resources we need to continue."

Harris held a conference call with staff on Tuesday afternoon to inform them of her decision, according to sources, who spoke on condition of anonymity.

Harris, 55, positioned herself as a unifying candidate who could energise the party's base of young, diverse progressives while also appealing to more moderate voters.

Yet after climbing into double digits in opinion polls following a strong debate performance in June, Harris slid out of the top tier in recent months and lags behind leading candidates' fundraising hauls.

The departure of the senator from California from the race is the first of a top-tier candidate from the crowded nominating contest.

Harris entered the race as an immediate front runner but then struggled to maintain support, which critics said was fuelled by her inability to articulate policy positions and the backlash of an attempt to attack her rival, former vice president Joe Biden.

Australian Associated Press