20 was the bare minimum to override the veto. If two-thirds of the House also overrides the veto, the bill will pass into law. The House overwhelmingly passed the original bill last summer, meaning there are good odds online sports betting will be legalized in Maine.
House vote could come as early as next Tuesday. Regulators will still need to approve any new online and retail sportsbooks, but assuming Maine follows other states' timelines, which have averaged roughly six months, sports betting (could) begin as early as 2020 football season.
Maine will (likely) become the first state in the country with untethered online sports betting and traditional retail gaming establishments. Commercial/tribal casinos, horse tracks, OTBs can all accept online bets, as can other third-party vendors not associated with them.