Clerics strongly opposed the nomination of two female judges to the Supreme Court.
The two main contenders nominated the former president after a divisive campaign.
The MDP is now the largest party with more than 50,000 members.
The by-election was called after the resignation of two council members.
MDP won an unprecedented super majority of the 87-member parliament.
MDP has taken a two-third majority of the 19th parliament.
The warning came after JP leader Gasim Ibrahim claimed the MDP would build temples and churches.
Millionaire lawmakers won't accept bribes, JP leader Gasim suggested.
The party has denied the allegations after reports of phone calls from campaign offices.
Hisaan Hussain was accused of mistreating five people who lived in her house.
The MDP candidate for the Mahibadhoo constituency dropped out.
The second MDP government will "show the difference" with a parliament majority, he said.
The attacks came after "constant death threats," Mickail Naseem said.
The MDP proposes a dramatic reduction in the powers of the Supreme Court.
The four-party coalition failed to agree on a formula for jointly contesting the elections in March.
The former actor-comedian plans to contest in an MDP primary to seek re-election on the party's ticket.
The MDP urged supporters to gather in Malé on November 1 to welcome its exiled leader.