WASHINGTON - Corey Lewandowski, an outside adviser to US President Donald Trump, said on Wednesday he believes former New York City Mayor Michael Bloomberg could pose a stiff challenge to the president's re-election in 2020 if Bloomberg were to run as a Democrat.Lewandowski, who remains in close touch with the president after serving as his campaign manager during part of the 2016 campaign, told reporters that Bloomberg has the advantage of not being a Washington insider, has the wealth to finance a campaign and holds policy positions popular among Democrats.Bloomberg is an "American success story" who would be "very competitive" but it is unclear whether he could beat the Republican incumbent in his expected 2020 re-election race, Lewandowski said.Bloomberg is the billionaire founder and chief executive of Bloomberg LP, the global media company. He has considering running for president in the past as an independent but eventually shied away. The Guardian newspaper reported earlier this month that Bloomberg, 76, is thinking of running in 2020 as a Democrat.Bloomberg was a registered Democrat but became a Republican before running for mayor of the nation's largest city in 2001. He left the Republican Party to become an independent in 2007 and served three terms as mayor, stepping down in 2013.At a breakfast session hosted by The Christian Science Monitor newspaper, Lewandowski also said he would recommend to Trump that he not sit for an interview with US Special Counsel Robert Mueller for Mueller's investigation into whether Russia colluded with the Trump presidential campaign.Trump and his lawyers are discussing privately the circumstances in which he would agree to provide testimony. Trump lawyer Rudy Giuliani has publicly expressed concerns that prosecutors might unfairly lure Trump into making false statements and expose himself to perjury charges."I don't see any benefit for him doing that," Lewandowski said of Trump testifying.He said Mueller should be pressed to conclude his investigation as quickly as possible and report on his findings by the Sept. 3 Labor Day holiday.