Former New York Mayor Michael Bloomberg is considering whether to jump into the Democratic primary, as he grows concerned that no one in the current field can beat President Trump.

Bloomberg, worth an estimated $52 billion, previously ruled out a run, saying it would be difficult to succeed in a crowded Democratic field.

But his political consultant, Howard Wolfson, tweeted on Thursday that he is having a change of heart.

“Mike believes that Donald Trump represents an unprecedented threat to our nation,” Wolfson wrote, noting that Bloomberg has spent more than $100 million to elect Democrats to Congress in 2018. “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.”

Bloomberg has not made a final decision. The New York Times, which first reported Bloomberg’s potential turnaround, said that he plans to take out papers to run in the Alabama primary, which has an early filing deadline.

Bloomberg, 77, served three terms as mayor, from 2002 through 2013. His successor, Bill de Blasio, already mounted a campaign for president and dropped out. Once a Republican, Bloomberg has previously mulled whether to run for president as an independent.

He is the co-founder and primary funder of Everytown for Gun Safety, which advocates for gun control measures. He has previously said that if he ran for president, he would make climate change his top issue.

Another billionare, Tom Steyer, is already running for the Democratic nomination, and is also focused on climate change.

On Thursday, Sen. Elizabeth Warren published an online calculator for billionaires to compute their tax bill under her wealth tax proposal. Bloomberg is one of three billionaires named on the site, along with Bill Gates and Leon Cooperman, both of whom have been critical of the plan. According to the calculator, Bloomberg would pay $3 billion annually under Warren’s plan.