United States Democrat Cory Booker has dropped out of the 2020 presidential race, ending a campaign whose message of unity and love failed to resonate in a political era marked by chaos and anxiety.

His departure on Monday now leaves a field that was once the most diverse in history with just one remaining African American candidate, former Massachusetts Governor Deval Patrick. The once two-dozen-strong field now has just 12 candidates vying for the democratic ticket.

Since launching his campaign last February, Booker, a US senator from New Jersey, struggled to raise the type of money required to support a White House bid. He was at the back of the pack in most surveys and failed to meet the polling requirements needed to participate in Tuesday's debate.

Booker also failed to qualify for last month's debate and exits the race polling in low single digits in the early primary states and nationwide.

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In an email to supporters, Booker said he "got into this race to win" and that his failure to make the debates prevented him from raising the money required for victory.

"Our campaign has reached the point where we need more money to scale up and continue building a campaign that can win - money we don't have, and money that is harder to raise because I won't be on the next debate stage and because the urgent business of impeachment will rightly be keeping me in Washington," he said.

Booker had warned that the looming impeachment trial of President Donald Trump would deal a "big, big blow" to his campaign by pulling him away from Iowa in the final weeks before the February 3 Iowa caucuses.

In his email to supporters, Booker pledged to do "everything in my power to elect the eventual Democratic nominee for president", though his campaign says he has no immediate plans to endorse a candidate in the primary.

Failure to connect

Once lauded by media executive and talk show host Oprah Winfrey as the "rock star mayor", Booker helped lead the renewal of Newark, New Jersey and became known for his headline-grabbing feats, including running into a burning building to save a woman. His rhetorical skills and Ivy League background often brought comparisons to former President Barack Obama.

It's with a full heart that I share this news—I'm suspending my campaign for president.



To my team, supporters, and everyone who gave me a shot—thank you. I am so proud of what we built, and I feel nothing but faith in what we can accomplish together. pic.twitter.com/Fxvc549vlJ — Cory Booker (@CoryBooker) January 13, 2020

But an early focus on building out a strong and seasoned campaign operation in Iowa and South Carolina may have hampered Booker's campaign in the long run, as the resources he spent early on staff there left him working with a tight budget in the later stages of the primary, when many of his opponents were going on air with television ads.

That meant that even later in the campaign, after he had collected some of the top endorsements in Iowa and visited South Carolina almost more than any other candidate, a significant portion of the electorate in both states either said they were unfamiliar with his campaign or viewed him unfavourably.

On the stump, Booker urged voters to use "radical love" to overcome what he considered Trump's hate. The candidate also touted his own bipartisan deal-making abilities.

However, he rarely drew a contrast with his Democratic opponents, even when asked directly. Some supporters worried that his message on Trump was not sharp enough to go up against a Republican president known for dragging his opponents into the mud.

Throughout the race, Booker also struggled to land on a message that would resonate with voters, despite long being seen as a progressive Democrat in the Senate, pushing for criminal justice reform and cannabis legalisation.

On the campaign trail, he proposed establishing a $1,000 savings account for every child born in the US to help close the racial wealth gap. He was also among the first candidates to release a gun control plan, which at the time was the most ambitious in the field, as it included a gun licensing programme that would have been seen as political suicide just a decade before.

Booker also released an early criminal justice reform plan that focused heavily on addressing sentencing disparities for drug crimes.

Cory, you campaigned with joy and heart, and instead of just talking about bringing people together, you did it every day. You made our politics better just by running. Grateful to you and looking forward to your continued leadership. — Joe Biden (@JoeBiden) January 13, 2020

Two frontrunners in the race, Senator Elizabeth Warren and former Vice President Joe Biden, tweeted their praise of Booker just minutes after his withdrawal announcement.

"Cory, you campaigned with joy and heart, and instead of just talking about bringing people together, you did it every day," wrote Biden.

Booker's Senate seat is up for a vote this year and he plans to run for re-election. Despite a handful of candidates already campaigning for the seat, Booker is expected to have an easy path to victory.