NEW YORK (BLOOMBERG) - Senator Kirsten Gillibrand is ending her campaign for the Democratic presidential nomination after failing to qualify for the third debate.

"Today, I am ending my campaign for president," she tweeted. "I think it's important to know how you can best serve," adding that her supporters should "go beat Donald Trump."

Gillibrand launched her campaign with a focus on women's issues. She rode the momentum of the #MeToo Movement that brought women to the polls in the 2018 midterms.

But in a presidential field containing the most female candidates ever, her pitch didn't catch on, and by the time she began embracing broader issues like campaign finance reform she had trouble finding her lane.

Only three hours before departing the race, Gillibrand tweeted a link to donate to her campaign, saying she was to push her campaign themes at the Sept 12 debate in Houston. But she failed to meet both the donor and polling qualification.

"There is dignity and strength in knowing when it's not your time. Moving forward, Kirsten will focus on uniting our party and our country to beat Donald Trump, flip the Senate and elect women up and down the ballot," said a statement from her campaign.

From the onset her campaign failed to gain the momentum expected from a senator holding the New York seat formerly occupied by Hillary Clinton. Throughout the race, Gillibrand hovered around 1 per cent in national polls.

She also struggled in fundraising, netting US$3 million (S$4 million) in the first quarter and US$2.3 million in the second. While she participated in the first two debates of Democratic candidates, she failed to make the cut for the third in Houston in September.

In August, Gillibrand told several news organisations that she would be open to serving as vice-president.

Gillibrand, 52, has been a senator since 2009, when she was appointed to the Senate by New York's governor after Clinton became secretary of State. Gillibrand won the seat in 2010 and was re-elected in 2016. Before entering the Senate she worked as a lawyer in a private practice until she was elected to the House of Representatives in 2006.

In the Senate, Gillibrand pushed for the repeal of "Don't Ask, Don't Tell" that was passed in 2010. She also was the first senator to call for fellow Democrat Al Franken to resign from the chamber after he was accused of sexual improprieties.