WASHINGTON — And then there were 12.

New Jersey Sen. Cory Booker suspended his campaign for the Democratic presidential nomination on Monday, ending a once-promising bid that never quite caught on with voters.

“Today I’m suspending my campaign for president with the same spirit with which it began: with my faith in us, my faith in us together as a nation, that we share a common pain and common problems that can only be solved with a common purpose and a sense of common cause,” Booker said in a video to supporters announcing his decision.

Booker’s exit leaves a dozen candidates still running for the Democratic nomination for president, roughly half the number that began in the historically large field.

Booker began the campaign viewed as a potential serious candidate for president. He’d been carefully laying the groundwork to run for years, assiduously courting donors and activists, and working feverishly to support other Democrats in down-ticket races.

His strong oratorical skills and upbeat, love-first message seemed like it might catch on with a base desperate to turn the page on President Trump’s unending fury and divisiveness. And his background — as a Rhodes Scholar and Stanford football player turned mayor of Newark, one of the country’s roughest cities — was an appealing one.

Booker actually led the field in Iowa lawmaker endorsements for much of the race, as local lawmakers saw a promising campaign. As one of the few black candidates in the field — and as one with a tendency toward sweeping, unity-oriented speeches — he’d long drawn comparisons to former President Obama.

But a historically crowded field and stringent debate qualification rules were hurdles Booker couldn’t overcome. He was forced to spend a bunch of time and money scrambling to find enough donors necessary to qualify for ever-increasing debate requirements. Even after he hit those benchmarks after months of begging for $1 donations, Booker struggled to hit the slowly increasing polling requirements to make the debate stage.

The final blow was likely when he failed to qualify for Tuesday night’s debate — the final one before Iowa’s influential caucuses in early February.

Booker’s positivity-focused, establishment-leaning bid may not have been suited to the times.

Biden continues to lead national polls based on his strength with moderate and non-white voters, two groups Booker was going to need to crack to make a move in the race, and Buttigieg’s rise blocked out Booker from winning moderate voters who wanted someone younger and more compelling than the former vice president.

Booker was squeezed on the left by Warren and Sanders, whose supporters view Booker’s long ties to the business community as anathema. And Democrats of all ideological stripes are just as angry at Trump as he is at them, so Booker’s sunny messaging may not have connected the way he’d hoped.

Booker’s exit makes a Democratic field that was once historic in its diversity even older and whiter.

Tuesday’s debate will be the first one with only white candidates: Former Vice President Joe Biden, billionaire Tom Steyer, South Bend Mayor Pete Buttigieg, and Sens. Bernie Sanders (I-Vt.) Elizabeth Warren (D-Mass.) and Amy Klobuchar (D-Minn.).

Former New York City Mayor Michael Bloomberg has spent enough money to emerge as a candidate who has to be taken seriously as well. With Booker out, the only black candidate left is former Massachusetts Gov. Deval Patrick, who has failed to break out in polling, and the only other candidates of color remaining in the race are businessman Andrew Yang and Rep. Tulsi Gabbard (D-Hawaii).

Booker lasted longer than 15 other candidates, including fellow Sens. Kirsten Gillibrand (D-N.Y.) and Kamala Harris (D-Calif.), former Secretary of Housing and Urban Development Julian Castro, former Washington Gov. Jay Inslee (D), Montana Gov. Steve Bullock (D), and former Rep. Beto O’Rourke (D-Texas).

Cover: Sen. Cory Booker (D-NJ) speaks during a television interview after the Democratic Presidential Debate at Tyler Perry Studios November 20, 2019 in Atlanta, Georgia. (Photo by Joe Raedle/Getty Images)