Moyes wants Mangala to replace Nemanja Vidic in what will be a much-changed back four at Old Trafford next season, but faces competition from Chelsea and Manchester City for the centre-back.
Mangala, 23, has a £35m buy-out clause in his Porto contract, but Moyes has been promised unlimited funds to rebuild the fading champions and is ready to make his move for a player tracked by former United boss Sir Alex Ferguson.
Mangala joined Porto in 2011 from Standard Liege in a £6m deal and has gone on to establish himself as one of the best young defenders in Europe.
Moyes will have huge holes to fill in his defence this summer, with Rio Ferdinand and Patrice Evra ready to follow Vidic out of Old Trafford.
Ferdinand is expected to retire, while Evra is set to return to France to join either Monaco or Paris St-Germain.
Vidic, meanwhile, is understood to gave agreed a two-year deal with Italian giants Inter Milan.
Written by Steve Milne @stevenmilnelive