Sen. Cory Booker dropped his Democratic presidential bid on Monday, leaving a dozen hopefuls in the race less than a month before the first caucuses in Iowa.

Booker, the former mayor of Newark, noted the problems he was having making a mark in the polls and raising funds in an e-mail sent to supporters.

“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,” Booker said.

Moments after Booker tweeted the news on Monday, President Trump mocked him.

“Really Big Breaking News (Kidding): Booker, who was in zero polling territory, just dropped out of the Democrat Presidential Primary Race,” the president tweeted. “Now I can rest easy tonight. I was sooo concerned that I would someday have to go head to head with him!”

In an interview with The Associated Press last week, Booker acknowledged he would be fighting an uphill battle.

“If we can’t raise more money in this final stretch, we won’t be able to do the things that other campaigns with more money can do to show presence,” he said.

Booker launched his presidential campaign last February, the first day of Black History Month, and with his departure, only one black candidate remains — former Massachusetts Gov. Deval Patrick.

In the fourth-quarter of last year, Booker’s campaign revealed that it raised $6.6 million, far less than the of other Democratic candidates.

Booker failed to qualify for the Democratic debate in December and didn’t hit the requirement for Tuesday’s debate in Iowa.

According to an average of national polls by RealClear Politics, Booker was polling at 1.8 percent, way behind the rest of the pack.

Booker is the third Democrat to drop out in January, along with Marianne Williamson and ex-HUD Secretary Julian Castro. Over the course of the last year, the field of Democratic candidates has been pared down from 29 to 12.

Meanwhile, Staten Island Rep. Max Rose, who is facing a well-funded Republican challenge to his re-election, endorsed Mike Bloomberg’s self-financed bid for the Democratic nomination. Rose is Bloomberg’s first congressional endorsement.

“He thinks Bloomberg will help with his re-election money,” a source told The Post.