Former New York mayor Michael Bloomberg is preparing to enter the crowded race to become the Democratic nominee for the 2020 presidential election, US media reported Thursday. The 77-year-old is expected to file paperwork in at least one state this week declaring himself a candidate, according to multiple outlets including The New York Times. Bloomberg had said back in March he wouldn't run, but has been toying for weeks with the idea of seeking the White House after all, according to an advisor, who was quoted as saying he had yet to make a final decision. The billionaire has, though, sent members of staff to Alabama to gather the necessary signatures required to register for that state's primary ahead of the deadline Friday in anticipation of a bid, the reports said. Alabama is not one of the earlier primaries but it has one of the earliest deadlines. The move is the first clear sign that Bloomberg, long touted as a possible US presidential candidate, is getting ready to battle it out to take on President Donald Trump. "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 advisor Howard Wolfson said in a statement. "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," Wolfson added, according to Bloomberg News. Bloomberg, the co-founder and CEO of the media company that shares his name, is one of the richest people in the United States. His huge personal wealth would likely shake up the contest at a time when frontrunner Joe Biden's fundraising is sagging. Bloomberg is perceived as a centrist figure close to Wall Street who views Elizabeth Warren and Bernie Sanders, two of the other Democratic frontrunners, as too left-wing. "He thinks (leader) Joe Biden is weak and Sanders and Warren can't win," the New York Post quoted a source familiar with Bloomberg's plans as saying. Bloomberg's entry would bloat an already crowded field of contenders, with 17 candidates currently vying for the right to take on Trump as the Democratic nominee. - 'Scared' - Bloomberg, who was elected mayor of the Big Apple in 2001 and served until 2013, is known to be opposed to some of the policies espoused by Warren and Sanders. They regularly lash out at financiers and big corporations. Both have said they plan to hit the rich with bigger taxes. "Welcome to the race, @MikeBloomberg!" tweeted Warren. "If you're looking for policy plans that will make a huge difference for working people and which are very popular, start here," she added. Warren accompanied the tweet with a link link to a calculator on her website which works out how much more billionaires would pay in tax under her presidency. "The billionaire class is scared and they should be scared," Sanders posted on Twitter, in a not-so-subtle reference to the reports about Bloomberg. Bloomberg has switched between the Republican and Democratic parties over the years and also served as an independent mayor. He has used some of his fortune to back Democratic politicians and fund policies that he believes in -- including the fight against climate change and anti-vaping efforts. Bloomberg considered running as an independent in 2016 but opted not to for fear of splitting the Democratic vote.