WASHINGTON - Fifty Senate lawmakers have endorsed a legislative measure to override the Federal Communications Commission’s recent decision to deregulate the broadband industry, top Democrats said Monday.The tally leaves supporters just one vote shy of the 51 required to pass a Senate resolution of disapproval, in a legislative gambit aimed at restoring the agency’s net neutrality rules.Those rules, which banned Internet providers from blocking or slowing down websites, were swept away in a December vote led by Republican FCC Chairman Ajit Pai.