Mike Bloomberg, the billionaire former mayor of New York, is preparing to enter the Democratic presidential primary, according to NBC News, which cited a Bloomberg advisor.

The move didn't necessarily mean Bloomberg, 77, was announcing a campaign, a source close to Bloomberg told NBC News. Rather, this source said, he's doing this to keep his options open. Bloomberg is "troubled" by what he has seen in the Democratic field, the source added.

"He's still not sure," a source told CNBC. This source also said these are "unprecedented times" and that Bloomberg is concerned about what he's seeing both from Democrats and President Donald Trump.

Howard Wolfson, a top advisor to Bloomberg, released a statement acknowledging Bloomberg's concern with the state of the race:

Mike believes that Donald Trump represents an unprecedented threat to our nation. In 2016, he spoke out at the Democratic Convention, warning against a Trump presidency. In 2018 he spent more than $100 million to help elect Democrats to ensure that Congress began to hold the President accountable. And this year he helped Democrats win control of both houses of the Virginia legislature. We now need to finish the job and ensure that Trump is defeated — but Mike is increasingly concerned that the current field of candidates is not well positioned to do that. If Mike runs he would offer a new choice to Democrats built on a unique record running America's biggest city, building a business from scratch and taking on some of America's toughest challenges as a high-impact philanthropist. Based on his record of accomplishment, leadership and his ability to bring people together to drive change, Mike would be able to take the fight to Trump and win.

Bloomberg has emerged as a force in Democratic politics, mostly because of his deep pockets. He spent more than $110 million backing Democrats during the 2018 congressional midterm elections. The gun control group he backs, Everytown for Gun Safety, spent $2.5 million in Virginia this year, helping the Democratic Party take control of the statehouse for the first time since 1994. Bloomberg earlier this year launched Beyond Carbon, a $500 million group that advocates for energy reform.

The National Rifle Association tweeted Thursday evening in response to the news of Bloomberg planning to enter the race: "Bloomberg bought Virginia on Tuesday and now he wants to buy America. His goal is to destroy #2A."

NRA tweet

CNBC previously reported that he was prepared to spend at least $100 million if he ran for president.

Bloomberg had been a registered Republican and independent, as well as a Democrat. He re-registered as a Democrat a month before the party took back the House of Representatives in the 2018 midterms.

The New York Times reported Thursday, citing people briefed on the plans, that Bloomberg was working on entering at least one Democratic presidential primary and potentially the overall race for the chance to take on Trump, 73, in 2020.

CNBC had reported last month that Bloomberg was discussing the possibility of a run as former Vice President Joe Biden, a fellow moderate, had shown signs of struggling against Sen. Elizabeth Warren, the Massachusetts liberal who is pushing for big tax hikes on the wealthy and expansive government programs such as "Medicare for All."

Biden, 76, leads Warren, 70, by under 8 percentage points in the Real Clear Politics national Democratic primary polling average. Sen. Bernie Sanders, the 78-year-old Vermont independent who identifies as a democratic socialist, is in third place. South Bend, Indiana, Mayor Pete Buttigieg, 37, is in fourth.

Warren — who has been publicly sparring with billionaires, including Bill Gates and Leon Cooperman, in recent days over her plans for a wealth tax — tweeted Thursday evening in response to the news.

Warren tweet

"More billionaires seeking more political power surely isn't the change America needs," Faiz Shakir, Bernie Sanders' campaign manager, told NBC.

There is already a billionaire in the Democratic primary, Tom Steyer, and he has yet to catch on with voters despite spending tons of money to try to break through. On Thursday, a Steyer aide apologized following an Associated Press report saying he had privately offered local Iowa politicians campaign contributions in exchange for their endorsements.

Representatives for the Biden and Buttigieg campaigns didn't immediately respond to a request for comment.

In polls of the early nominating states Iowa and New Hampshire, which are seen as crucial for generating momentum in the race, Biden often trails Warren and Sanders.

One advisor told the Times that Bloomberg had not made a decision, but he has sent aides to Alabama, which has a Friday deadline for candidates to enter the race.

Along with California and Texas, Alabama is one of the states scheduled to vote on the pivotal "Super Tuesday" primary day March 3 next year. The Iowa and Nevada caucuses and the New Hampshire and South Carolina primaries are scheduled for February.

Bloomberg has previously flirted with running for president in several election cycles. Earlier this year he had said he would not seek the White House.